Algebra Quiz: Deriving Applying The Geometric Series Formula

What this quiz covers

This quiz focuses on Deriving Applying The Geometric Series Formula, giving you a quick way to practice the rules, question types, and explanations that matter most for Algebra.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. $S_n = \frac{a(1-r^n)}{1-r}$ (correct answer)
2. $S_n = a + (n-1)r$
3. $S_n = \frac{a(1-r^{n-1})}{1-r}$
4. $S_n = \frac{a(1-r^n)}{1+r}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula $S_n = a(1 - r^n)/(1 - r)$ to calculate these sums efficiently. The geometric series formula $S_n = a(1 - r^n)/(1 - r)$ comes from a clever trick: write the sum $S_n = a + ar + ar^2 + \cdots + ar^{n-1}$, then multiply by $r$ to get $rS_n = ar + ar^2 + ar^3 + \cdots + ar^n$. Subtracting these ($S_n - rS_n$) makes almost all terms cancel, leaving just $S_n(1 - r) = a(1 - r^n)$, so $S_n = a(1 - r^n)/(1 - r)$. The middle terms canceling is the magic that makes this work! Deriving $S_n = a(1 - r^n)/(1 - r)$: (1) Write the sum: $S_n = a + ar + ar^2 + \cdots + ar^{n-1}$. (2) Multiply both sides by $r$: $rS_n = ar + ar^2 + ar^3 + \cdots + ar^n$. (3) Subtract second from first: $S_n - rS_n = (a + ar + ar^2 + \cdots + ar^{n-1}) - (ar + ar^2 + ar^3 + \cdots + ar^n)$. (4) Notice the cancellation: all middle terms cancel, leaving $S_n - rS_n = a - ar^n$. (5) Factor left side: $S_n(1 - r) = a(1 - r^n)$. (6) Divide by $(1 - r)$: $S_n = a(1 - r^n)/(1 - r)$. This derivation shows why the formula works—it's not just memorization! Choice B correctly derives the formula through the subtraction method with the accurate final expression. Choice A makes an error in the derivation by using $r^{n-1}$ instead of $r^n$ in the numerator; this happens if you miss that the remaining term after cancellation is $-ar^n$ from the shifted series. Derivation memory aid: the trick is writing $S_n$, then writing $rS_n$ (shifted one term), then subtracting. When you subtract, the middle terms align and cancel: $S_n$ has 'ar' and $rS_n$ has 'ar' (opposite signs, cancel!), $S_n$ has 'ar^2$and$rS_nhas 'ar^2' (cancel!), etc. Only a fromS_n$and$-ar^n$from$rS_n$ don't cancel. This telescoping is the insight! Once you see it, you'll never forget the derivation.

Question 2

1. $a=2, r=2, n=4; S_4=30$
2. $a=2, r=3, n=4; S_4=80$ (correct answer)
3. $a=2, r=3, n=3; S_3=26$
4. $a=6, r=3, n=4; S_4=240$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula $S_n = \frac{a(1 - r^n)}{1 - r}$ to calculate these sums efficiently. A geometric series is the sum of terms from a geometric sequence: if the sequence is 2, 6, 18, 54, ... (multiply by 3 each time), the series is 2 + 6 + 18 + 54 + ... (adding those terms up). Rather than adding manually (tedious for many terms!), we use the formula $S_n = \frac{a(1 - r^n)}{1 - r}$, where a is the first term, r is the common ratio, and n is how many terms we're summing. This formula works for any finite geometric series! Let's verify the formula works for a simple example: series 2 + 6 + 18 + 54 with a = 2, r = 3, n = 4. Formula: $S_4 = 2(1 - 3^4)/(1 - 3) = 2(1 - 81)/(-2) = 2(-80)/(-2) = 80$. Manual addition: 2 + 6 + 18 + 54 = 80 ✓. The formula gives the same answer as adding manually, but it's much faster for large n! Choice A correctly identifies a=2, r=3, n=4 and calculates the sum as 80 with accurate arithmetic. Choice B identifies the parameters wrong: n=3 instead of n=4; getting a, r, or n wrong throws off the entire calculation! Why this formula is powerful: to sum 2 + 6 + 18 + 54 + ... + (2·3^99), you'd need to add 100 terms manually (impossible!). With the formula: $S_100 = 2(1 - 3^{100})/(1 - 3)$ and you're done (calculator handles 3^100). The formula turns a hundred-operation problem into a few operations. That's the beauty of having a formula!

Question 3

1. $242$ (correct answer)
2. $80$
3. $121$
4. $244$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. A geometric series is the sum of terms from a geometric sequence: if the sequence is 2, 6, 18, 54, ... (multiply by 3 each time), the series is 2 + 6 + 18 + 54 + ... (adding those terms up). Rather than adding manually (tedious for many terms!), we use the formula S_n = a(1 - r^n)/(1 - r), where a is the first term, r is the common ratio, and n is how many terms we're summing. This formula works for any finite geometric series! For the series 2 + 6 + 18 + 54 + 162 with a = 2, r = 3, n = 5: Using the formula S_n = a(1 - r^n)/(1 - r), we substitute: S_5 = 2(1 - 3^5)/(1 - 3) = 2(1 - 243)/(-2) = 2(-242)/(-2) = 2(121) = 242. So the sum of the first 5 terms is 242. Choice A correctly applies the formula with a = 2, r = 3, n = 5 and calculates the sum as 242 with accurate arithmetic. Choice C gives 121, which is exactly half of the correct answer 242. This suggests forgetting to multiply by a = 2 in the final step: (1 - 3^5)/(1 - 3) = (-242)/(-2) = 121, but we need 2 × 121 = 242. Don't forget the first term multiplier a in the formula! Let's verify the formula works for a simple example: series 2 + 6 + 18 + 54 with a = 2, r = 3, n = 4. Formula: S_4 = 2(1 - 3^4)/(1 - 3) = 2(1 - 81)/(-2) = 2(-80)/(-2) = 80. Manual addition: 2 + 6 + 18 + 54 = 80 ✓. The formula gives the same answer as adding manually, but it's much faster for large n!

Question 4

1. $\dfrac{10(1-0.8^5)}{1-0.8}=33.616$ (correct answer)
2. $\dfrac{10(1-0.8^4)}{1-0.8}=29.52$
3. $\dfrac{10(1-0.8^5)}{1-0.8}=16.808$
4. $10(0.8)^5=3.2768$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. Geometric series appear in finance: when calculating mortgage payments, the total amount paid is a geometric series with the interest rate as the ratio. Each payment accumulates interest, creating the geometric pattern. The formula lets you calculate loan payoffs, investment growth with regular contributions, and other real-world money situations where compounding occurs! In the context of a bouncing ball, each bounce height is 80% of the previous: 10 ft, then 10(0.8) = 8 ft, then 10(0.8)² = 6.4 ft, creating a geometric series. With a = 10, r = 0.8, n = 5, the total is S_5 = 10(1 - (0.8)^5)/(1 - 0.8) = 10(1 - 0.32768)/(0.2) = 10(0.67232)/(0.2) = 10(3.3616) = 33.616. This shows the total distance traveled upward in 5 bounces. Choice A correctly applies the formula with a = 10, r = 0.8, n = 5 and calculates the sum as 33.616 with accurate arithmetic. Choice C has a calculation error: it shows 16.808 which is exactly half of 33.616, suggesting they might have divided by 0.2 incorrectly or made an arithmetic mistake. When 0 < r < 1, both (1 - r^n) and (1 - r) are positive, so the sum should be positive and match our calculation. Why this formula is powerful: to sum 2 + 6 + 18 + 54 + ... + (2·3^99), you'd need to add 100 terms manually (impossible!). With the formula: S_100 = 2(1 - 3^100)/(1 - 3) and you're done (calculator handles 3^100). The formula turns a hundred-operation problem into a few operations. That's the beauty of having a formula!

Question 5

1. $\dfrac{3\left(1-1.1^{9}\right)}{1-1.1}$
2. $\dfrac{3\left(1-10\cdot 1.1\right)}{1-1.1}$
3. $\dfrac{3\left(1-1.1^{10}\right)}{1-1.1}$ (correct answer)
4. $\dfrac{3\left(1-1.1^{10}\right)}{1.1}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. To use the formula: (1) identify the first term a (what's the first number being added?), (2) find the common ratio r (divide any term by the previous term), (3) count how many terms n you're summing, (4) substitute into S_n = a(1 - r^n)/(1 - r), (5) calculate carefully. Example: for 5 + 10 + 20 + 40 + 80, we have a = 5, r = 2, n = 5, so S_5 = 5(1 - 2^5)/(1 - 2) = 5(1 - 32)/(-1) = 5(-31)/(-1) = 155. For the series 3 + 3(1.1) + 3(1.1)² + ... + 3(1.1)⁹ with a = 3, r = 1.1, n = 10: Using the formula S_n = a(1 - r^n)/(1 - r), we substitute: S_10 = 3(1 - 1.1^10)/(1 - 1.1). This matches the expression in choice A exactly. So the sum is expressed as 3(1 - 1.1^10)/(1 - 1.1). Choice A correctly applies the formula with a = 3, r = 1.1, n = 10 with accurate substitution. Choice B has n = 9 instead of n = 10: when counting from 3(1.1)⁰ to 3(1.1)⁹, that's 10 terms total (including the 0th power). Remember that powers go from 0 to n-1, giving n terms total. Off-by-one errors in counting terms are common! Common pitfall: confusing n (number of terms) with the last exponent. In a + ar + ar² + ... + ar^(n-1), there are n terms, but the last exponent is (n-1)! Count carefully: if the series is 'first 5 terms,' then n = 5, but the last term is ar^4, not ar^5. Off-by-one errors in n throw off the sum. The formula already accounts for this with r^n, not r^(n-1).

Question 6

1. $\approx 33.62$ (correct answer)
2. $\approx 40.00$
3. $\approx 26.89$
4. $\approx 36.16$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. Geometric series appear in finance: when calculating mortgage payments, the total amount paid is a geometric series with the interest rate as the ratio. Each payment accumulates interest, creating the geometric pattern. The formula lets you calculate loan payoffs, investment growth with regular contributions, and other real-world money situations where compounding occurs! In the context of ball rebound heights, each height is 80% of the previous, creating a geometric series: 10 + 10(0.8) + 10(0.8)^2 + 10(0.8)^3 + 10(0.8)^4. With a = 10, r = 0.8, n = 5, the total is S_5 = 10(1 - 0.8^5)/(1 - 0.8) ≈ 10(1 - 0.32768)/0.2 ≈ 10(0.67232)/0.2 ≈ 10(3.3616) ≈ 33.62. This shows the total height summed over the first 5 bounces. Choice A correctly applies the formula with a = 10, r = 0.8, n = 5 and calculates the sum as ≈33.62 with accurate arithmetic. Choice D uses the arithmetic series formula instead of the geometric series formula: arithmetic series use S_n = n(a + last term)/2 (for constant difference), but this is a geometric series with constant ratio, needing S_n = a(1 - r^n)/(1 - r). Don't confuse the two types! For financial applications like mortgages: the payment P, interest rate r (per period), and number of periods n plug into formulas built from geometric series. You might see S = P((1+r)^n - 1)/r or similar—these come from rearranging the geometric series formula! Understanding the underlying geometric series helps you understand why mortgage formulas look the way they do.

Question 7

1. \1{,}200.00$
2. \1{,}236.94$ (correct answer)
3. \1{,}260.00$
4. \1{,}230.79$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. Geometric series appear in finance: when calculating mortgage payments, the total amount paid is a geometric series with the interest rate as the ratio. Each payment accumulates interest, creating the geometric pattern. The formula lets you calculate loan payoffs, investment growth with regular contributions, and other real-world money situations where compounding occurs! In the context of this savings plan, each payment of $100 grows by 0.5$1,236.94. Choice C correctly calculates the sum as $1,236.94 with accurate arithmetic and proper handling of the compound interest. Choice B gives$1,230.79, which might result from using 11 terms instead of 12, or from a calculation error. Remember that deposits from month 1 to month 12 means 12 deposits total, each earning different amounts of interest based on how long they've been in the account. For financial applications like mortgages: the payment P, interest rate r (per period), and number of periods n plug into formulas built from geometric series. You might see S = P((1+r)^n - 1)/r or similar—these come from rearranging the geometric series formula! Understanding the underlying geometric series helps you understand why mortgage formulas look the way they do.

Question 8

1. $\dfrac{120}{16}$
2. $\dfrac{1799}{128}$
3. $\dfrac{119}{16}$ (correct answer)
4. $\dfrac{63}{8}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. A geometric series is the sum of terms from a geometric sequence: if the sequence is 2, 6, 18, 54, ... (multiply by 3 each time), the series is 2 + 6 + 18 + 54 + ... (adding those terms up). Rather than adding manually (tedious for many terms!), we use the formula S_n = a(1 - r^n)/(1 - r), where a is the first term, r is the common ratio, and n is how many terms we're summing. This formula works for any finite geometric series! For the series 7 + 7(-0.5) + 7(-0.5)^2 + ... + 7(-0.5)^7 with a = 7, r = -0.5, n = 8: Using the formula S_n = a(1 - r^n)/(1 - r), we substitute: S_8 = 7(1 - (-0.5)^8)/(1 - (-0.5)) = 7(1 - 1/256)/(1.5) = 7(255/256)/(3/2) = 7(255/256)(2/3) = 7(510/768) = 3570/768 = 595/128 = 119/16. So the sum of the first 8 terms is 119/16. Choice B correctly applies the formula with a = 7, r = -0.5, n = 8 and calculates the sum as 119/16 with accurate arithmetic. Choice A gives 1799/128, which appears to use a different calculation—perhaps an error in computing (-0.5)^8 = 1/256 or in the subsequent arithmetic; with negative ratios, sign errors can compound quickly. Common pitfall: confusing n (number of terms) with the last exponent. In a + ar + ar² + ... + ar^(n-1), there are n terms, but the last exponent is (n-1)! Count carefully: if the series is 'first 5 terms,' then n = 5, but the last term is ar^4, not ar^5. Off-by-one errors in n throw off the sum. The formula already accounts for this with r^n, not r^(n-1).

Question 9

1. $\displaystyle 3\,\frac{1-1.1^{10}}{1-1.1}$ (correct answer)
2. $\displaystyle 3\,\frac{1-1.1^{10}}{1+1.1}$
3. $\displaystyle 3\,\frac{1-1.1^{9}}{1-1.1}$
4. $\displaystyle 3\,\frac{1.1^{10}-1}{1-1.1}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula $S_n = a(1 - r^n)/(1 - r)$ to calculate these sums efficiently. To use the formula: (1) identify the first term a (what's the first number being added?), (2) find the common ratio r (divide any term by the previous term), (3) count how many terms n you're summing, (4) substitute into $S_n = a(1 - r^n)/(1 - r)$, (5) calculate carefully. Example: for 5 + 10 + 20 + 40 + 80, we have a = 5, r = 2, n = 5, so $S_5 = 5(1 - 2^5)/(1 - 2) = 5(1 - 32)/(-1) = 5(-31)/(-1) = 155$. For the series 3 + 3(1.1) + 3(1.1)^2 + ... + 3(1.1)^9 with a = 3, r = 1.1, n = 10: Using the formula $S_n = a(1 - r^n)/(1 - r)$, we substitute: $S_10 = 3(1 - 1.1^{10})/(1 - 1.1)$, which is the expression needed. So the sum of the first 10 terms is given by that formula. Choice A correctly applies the formula with a = 3, r = 1.1, n = 10 with accurate parameters. Choice B identifies the parameters wrong by using n=9 instead of n=10; the exponents go from 0 to 9, which is 10 terms, so n=10, not n-1. Common pitfall: confusing n (number of terms) with the last exponent. In $a + ar + ar^2 + \dots + ar^{n-1}$, there are n terms, but the last exponent is (n-1)! Count carefully: if the series is 'first 10 terms,' then n = 10, but the last term is ar^9, not ar^{10}. The formula already accounts for this with r^n, not r^(n-1).

Question 10

1. $896$
2. $1024$
3. $1792$
4. $1785$ (correct answer)

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. To use the formula: (1) identify the first term a (what's the first number being added?), (2) find the common ratio r (divide any term by the previous term), (3) count how many terms n you're summing, (4) substitute into S_n = a(1 - r^n)/(1 - r), (5) calculate carefully. For the series 7 + 14 + 28 + ... with a = 7, r = 2, n = 8: Using the formula S_8 = 7(1 - 2^8)/(1 - 2) = 7(1 - 256)/(-1) = 7(-255)/(-1) = 7(255) = 1785. So the sum of the first 8 terms is 1785. Choice B correctly applies the formula with a = 7, r = 2, n = 8 and calculates the sum as 1785 with accurate arithmetic. Choice A forgets the (1 - r^n) is negative when r > 1: if r = 2 and n = 8, then r^n = 256, so (1 - 256) = -255 (negative!). Combined with (1 - r) = 1 - 2 = -1 (also negative), we get (-255)/(-1) = 255, then *7=1785 (positive). Two negatives make a positive—don't lose those signs! Why this formula is powerful: to sum 2 + 6 + 18 + 54 + ... + (2·3^99), you'd need to add 100 terms manually (impossible!). With the formula: S_100 = 2(1 - 3^100)/(1 - 3) and you're done (calculator handles 3^100). The formula turns a hundred-operation problem into a few operations. That's the beauty of having a formula!

Question 11

1. $80$ (correct answer)
2. $78$
3. $72$
4. $162$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula $S_n = a(1 - r^n)/(1 - r)$ to calculate these sums efficiently. A geometric series is the sum of terms from a geometric sequence: if the sequence is $2, 6, 18, 54, \dots$ (multiply by 3 each time), the series is $2 + 6 + 18 + 54 + \dots$ (adding those terms up). Rather than adding manually (tedious for many terms!), we use the formula $S_n = a(1 - r^n)/(1 - r)$, where a is the first term, r is the common ratio, and n is how many terms we're summing. This formula works for any finite geometric series! Let's verify the formula works for a simple example: series $2 + 6 + 18 + 54$ with a = 2, r = 3, n = 4. Formula: $S_4 = 2(1 - 3^4)/(1 - 3) = 2(1 - 81)/(-2) = 2(-80)/(-2) = 80$. Manual addition: $2 + 6 + 18 + 54 = 80$ ✓. The formula gives the same answer as adding manually, but it's much faster for large n! Choice B correctly applies the formula with a = 2, r = 3, n = 4 and calculates the sum as 80 with accurate arithmetic. Choice A identifies the parameters wrong: the common ratio r is found by dividing consecutive terms: $6/2 = 3$, not something else, and getting a, r, or n wrong throws off the entire calculation. Common pitfall: confusing n (number of terms) with the last exponent. In $a + ar + ar^2 + \dots + ar^{n-1}$, there are n terms, but the last exponent is (n-1)! Count carefully: if the series is 'first 5 terms,' then n = 5, but the last term is $ar^4$, not $ar^5$. Off-by-one errors in n throw off the sum. The formula already accounts for this with $r^n$, not $r^{n-1}$.

Question 12

1. $S_n=\frac{a(1-r)}{1-r^n}$
2. $S_n=\frac{a(1-r^{n-1})}{1-r}$
3. $S_n=\frac{a(1-r^n)}{1-r}$ (correct answer)
4. $S_n=a\,r^n$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula $S_n = a(1 - r^n)/(1 - r)$ to calculate these sums efficiently. The geometric series formula $S_n = a(1 - r^n)/(1 - r)$ comes from a clever trick: write the sum $S_n = a + ar + ar^2 + \cdots + ar^{n-1}$, then multiply by $r$ to get $rS_n = ar + ar^2 + ar^3 + \cdots + ar^n$. Subtracting these ($S_n - rS_n$) makes almost all terms cancel, leaving just $S_n(1 - r) = a(1 - r^n)$, so $S_n = a(1 - r^n)/(1 - r)$. The middle terms canceling is the magic that makes this work! Deriving $S_n = a(1 - r^n)/(1 - r)$: (1) Write the sum: $S_n = a + ar + ar^2 + \cdots + ar^{n-1}$. (2) Multiply both sides by $r$: $rS_n = ar + ar^2 + ar^3 + \cdots + ar^n$. (3) Subtract second from first: $S_n - rS_n = (a + ar + ar^2 + \cdots + ar^{n-1}) - (ar + ar^2 + ar^3 + \cdots + ar^n)$. (4) Notice the cancellation: all middle terms cancel, leaving $S_n - rS_n = a - ar^n$. (5) Factor left side: $S_n(1 - r) = a(1 - r^n)$. (6) Divide by $(1 - r)$: $S_n = a(1 - r^n)/(1 - r)$. This derivation shows why the formula works—it's not just memorization! Choice B correctly derives the formula through the subtraction method with accurate cancellation leading to $a(1 - r^n)/(1 - r)$. Choice A makes an error in the derivation: when subtracting $S_n - rS_n$, the middle terms should cancel, but this choice uses $r^{n-1}$ instead of $r^n$, missing the correct last term cancellation. Derivation memory aid: the trick is writing $S_n$, then writing $rS_n$ (shifted one term), then subtracting. When you subtract, the middle terms align and cancel: $S_n$ has 'ar' and $rS_n$ has 'ar' (opposite signs, cancel!), $S_n$ has 'ar^2$and$rS_nhas 'ar^2' (cancel!), etc. Only a fromS_n$and$ar^n$from$rS_n$ don't cancel. This telescoping is the insight! Once you see it, you'll never forget the derivation.

Question 13

1. $90$
2. $93$ (correct answer)
3. $96$
4. $189$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula $S_n = a(1 - r^n)/(1 - r)$ to calculate these sums efficiently. Let's verify the formula works for a simple example: series 2 + 6 + 18 + 54 with a = 2, r = 3, n = 4. Formula: $S_4 = 2(1 - 3^4)/(1 - 3) = 2(1 - 81)/(-2) = 2(-80)/(-2) = 80$. Manual addition: 2 + 6 + 18 + 54 = 80 ✓. The formula gives the same answer as adding manually, but it's much faster for large n! For the series 3 + 6 + 12 + 24 + 48 with a = 3, r = 2, n = 5: Using the formula $S_5 = 3(1 - 2^5)/(1 - 2) = 3(1 - 32)/(-1) = 3(-31)/(-1) = 3(31) = 93$. So the sum of the 5 terms is 93. Choice B correctly applies the formula with a = 3, r = 2, n = 5 and calculates the sum as 93 with accurate arithmetic. Choice D has a sign error: when r > 1, we have negative numerator and negative denominator: (1 - r^n) is negative (1 - big number), and (1 - r) is negative (1 - bigger than 1), so negative/negative = positive. This choice perhaps doubled or miscounted without signs. Track signs carefully! The three-step geometric series strategy: (1) Identify a, r, n: first term, common ratio (divide consecutive terms to find it), number of terms. (2) Check r ≠ 1 (if r = 1, sum = na instead). (3) Substitute into $S_n = a(1 - r^n)/(1 - r)$ and calculate carefully, watching signs: when r > 1, both (1 - r^n) and (1 - r) are negative; when 0 < r < 1, both are positive. The formula handles all cases!

Question 14

1. $315$ (correct answer)
2. $320$
3. $310$
4. $160$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. To use the formula: (1) identify the first term a (what's the first number being added?), (2) find the common ratio r (divide any term by the previous term), (3) count how many terms n you're summing, (4) substitute into S_n = a(1 - r^n)/(1 - r), (5) calculate carefully. For the series 5 + 10 + 20 + 40 + ... with a = 5, r = 2, n = 6: Using the formula S_6 = 5(1 - 2^6)/(1 - 2), we substitute: S_6 = 5(1 - 64)/(-1) = 5(-63)/(-1) = 5(63) = 315. Choice A correctly applies the formula with a = 5, r = 2, n = 6 and calculates the sum as 315 with accurate arithmetic. Choice D has a sign error: when r > 1, we have negative numerator and negative denominator: (1 - r^n) is negative (1 - big number), and (1 - r) is negative (1 - bigger than 1), so negative/negative = positive, but this choice likely forgot the signs and got half the value or similar. The three-step geometric series strategy: (1) Identify a, r, n: first term, common ratio (divide consecutive terms to find it), number of terms. (2) Check r ≠ 1 (if r = 1, sum = na instead). (3) Substitute into S_n = a(1 - r^n)/(1 - r) and calculate carefully, watching signs: when r > 1, both (1 - r^n) and (1 - r) are negative; when 0 < r < 1, both are positive. The formula handles all cases!

Question 15

1. $S_n-rS_n=a-ar^n$ (correct answer)
2. $rS_n-S_n=a-ar^n$
3. $S_n-rS_n=a-ar^{n-1}$
4. $S_n+rS_n=a+ar^n$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. The geometric series formula S_n = a(1 - r^n)/(1 - r) comes from a clever trick: write the sum S_n = a + ar + ar² + ... + ar^(n-1), then multiply by r to get rS_n = ar + ar² + ar³ + ... + ar^n. Subtracting these (S_n - rS_n) makes almost all terms cancel, leaving just S_n(1 - r) = a(1 - r^n), so S_n = a(1 - r^n)/(1 - r). The middle terms canceling is the magic that makes this work! Deriving S_n = a(1 - r^n)/(1 - r): (1) Write the sum: S_n = a + ar + ar² + ... + ar^(n-1). (2) Multiply both sides by r: rS_n = ar + ar² + ar³ + ... + ar^n. (3) Subtract second from first: S_n - rS_n = (a + ar + ar² + ... + ar^(n-1)) - (ar + ar² + ar³ + ... + ar^n). (4) Notice the cancellation: all middle terms cancel, leaving S_n - rS_n = a - ar^n. (5) Factor left side: S_n(1 - r) = a(1 - r^n). (6) Divide by (1 - r): S_n = a(1 - r^n)/(1 - r). This derivation shows why the formula works—it's not just memorization! Choice B correctly derives the formula through the subtraction method by showing S_n - rS_n = a - ar^n after cancellation. Choice A makes an error in the derivation: when subtracting S_n - rS_n, the middle terms should cancel, leaving a - ar^n, but this choice has a - ar^{n-1}, missing the correct exponent on the last term. Derivation memory aid: the trick is writing S_n, then writing rS_n (shifted one term), then subtracting. When you subtract, the middle terms align and cancel: S_n has 'ar' and rS_n has 'ar' (opposite signs, cancel!), S_n has 'ar²' and rS_n has 'ar²' (cancel!), etc. Only a from S_n and ar^n from rS_n don't cancel. This telescoping is the insight! Once you see it, you'll never forget the derivation.

Question 16

1. $\displaystyle \frac{128}{255}$
2. $\displaystyle \frac{63}{32}$
3. $\displaystyle \frac{255}{128}$ (correct answer)
4. $\displaystyle \frac{127}{64}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula $S_n = a(1 - r^n)/(1 - r)$ to calculate these sums efficiently. To use the formula: (1) identify the first term $a$ (what's the first number being added?), (2) find the common ratio $r$ (divide any term by the previous term), (3) count how many terms $n$ you're summing, (4) substitute into $S_n = a(1 - r^n)/(1 - r)$, (5) calculate carefully. Example: for $5 + 10 + 20 + 40 + 80$, we have $a = 5$, $r = 2$, $n = 5$, so $S_5 = 5(1 - 2^5)/(1 - 2) = 5(1 - 32)/(-1) = 5(-31)/(-1) = 155$. For the series $1 + \frac{1}{2} + \frac{1}{4} + \frac{1}{8} + \cdots$ (first 8 terms) with $a = 1$, $r = \frac{1}{2}$, $n = 8$: Using the formula $S_n = a(1 - r^n)/(1 - r)$, we substitute: $S_8 = 1(1 - (\frac{1}{2})^8)/(1 - \frac{1}{2}) = (1 - \frac{1}{256})/(\frac{1}{2}) = (\frac{255}{256})/(\frac{1}{2}) = \frac{255}{256} \times 2 = \frac{255}{128}$. So the sum of the first 8 terms is $\frac{255}{128}$. Choice A correctly applies the formula with $a = 1$, $r = \frac{1}{2}$, $n = 8$ and calculates the sum as $\frac{255}{128}$ with accurate arithmetic. Choice B has a sign error or miscalculation: it might come from using $(1 - r^n)/(r - 1)$ without adjusting signs properly; track signs carefully! The three-step geometric series strategy: (1) Identify $a$, $r$, $n$: first term, common ratio (divide consecutive terms to find it), number of terms. (2) Check $r \neq 1$ (if $r = 1$, sum = $n a$ instead). (3) Substitute into $S_n = a(1 - r^n)/(1 - r)$ and calculate carefully, watching signs: when $r > 1$, both $(1 - r^n)$ and $(1 - r)$ are negative; when $0 < r < 1$, both are positive. The formula handles all cases!

Question 17

1. $S_n - rS_n = a + ar^n$
2. $S_n - rS_n = a - ar^n$ (correct answer)
3. $S_n - rS_n = ar^n - a$
4. $S_n - rS_n = ar - ar^{n-1}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula $S_n = a(1 - r^n)/(1 - r)$ to calculate these sums efficiently. The geometric series formula $S_n = a(1 - r^n)/(1 - r)$ comes from a clever trick: write the sum $S_n = a + ar + ar^2 + \cdots + ar^{n-1}$, then multiply by $r$ to get $rS_n = ar + ar^2 + ar^3 + \cdots + ar^n$. Subtracting these ($S_n - rS_n$) makes almost all terms cancel, leaving just $S_n(1 - r) = a(1 - r^n)$, so $S_n = a(1 - r^n)/(1 - r)$. The middle terms canceling is the magic that makes this work! Deriving $S_n = a(1 - r^n)/(1 - r)$: (1) Write the sum: $S_n = a + ar + ar^2 + \cdots + ar^{n-1}$. (2) Multiply both sides by $r$: $rS_n = ar + ar^2 + ar^3 + \cdots + ar^n$. (3) Subtract second from first: $S_n - rS_n = (a + ar + ar^2 + \cdots + ar^{n-1}) - (ar + ar^2 + ar^3 + \cdots + ar^n)$. (4) Notice the cancellation: all middle terms cancel, leaving $S_n - rS_n = a - ar^n$. (5) Factor left side: $S_n(1 - r) = a(1 - r^n)$. (6) Divide by $(1 - r)$: $S_n = a(1 - r^n)/(1 - r)$. This derivation shows why the formula works—it's not just memorization! Choice A correctly derives the formula through the subtraction method by obtaining the key cancellation equation $S_n - rS_n = a - ar^n$. Choice B makes an error in the derivation: when subtracting $S_n - rS_n$, the middle terms should cancel, but this choice doesn't cancel correctly and misses the $a - ar^n$ result. Derivation memory aid: the trick is writing $S_n$, then writing $rS_n$ (shifted one term), then subtracting. When you subtract, the middle terms align and cancel: $S_n$ has 'ar' and $rS_n$ has 'ar' (opposite signs, cancel!), $S_n$ has 'ar^2' and $rS_n$ has 'ar^2$(cancel!), etc. Only a from$S_n$and$-ar^n$from$rS_n$ don't cancel. This telescoping is the insight! Once you see it, you'll never forget the derivation.

Question 18

1. $315$ (correct answer)
2. $155$
3. $320$
4. $310$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula $S_n = a(1 - r^n)/(1 - r)$ to calculate these sums efficiently. To use the formula: (1) identify the first term a (what's the first number being added?), (2) find the common ratio r (divide any term by the previous term), (3) count how many terms n you're summing, (4) substitute into $S_n = a(1 - r^n)/(1 - r)$, (5) calculate carefully. Example: for 5 + 10 + 20 + 40 + 80, we have a = 5, r = 2, n = 5, so $S_5 = 5(1 - 2^5)/(1 - 2) = 5(1 - 32)/(-1) = 5(-31)/(-1) = 155$. For the series 5 + 10 + 20 + 40 + 80 + 160 with a = 5, r = 2, n = 6: Using the formula $S_n = a(1 - r^n)/(1 - r)$, we substitute: $S_6 = 5(1 - 2^6)/(1 - 2) = 5(1 - 64)/(-1) = 5(-63)/(-1) = 315$. So the sum of the first 6 terms is 315. Choice A correctly applies the formula with a = 5, r = 2, n = 6 and calculates the sum as 315 with accurate arithmetic. Choice D calculates 155, which is the sum for n=5 instead of n=6; always double-check the number of terms to avoid off-by-one errors! The three-step geometric series strategy: (1) Identify a, r, n: first term, common ratio (divide consecutive terms to find it), number of terms. (2) Check $r \neq 1$ (if r = 1, sum = na instead). (3) Substitute into $S_n = a(1 - r^n)/(1 - r)$ and calculate carefully, watching signs: when r > 1, both (1 - r^n) and (1 - r) are negative; when 0 < r < 1, both are positive. The formula handles all cases!

Question 19

1. $\displaystyle 10\,\frac{1-0.8^5}{1-0.8}=33.616$ (correct answer)
2. $\displaystyle 10\,\frac{1-0.8^4}{1-0.8}=29.52$
3. $\displaystyle 10\,\frac{1-0.8^5}{1+0.8}=18.6756$
4. $\displaystyle 10\,\frac{1-0.8^5}{0.8}=8.404$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula $S_n = \frac{a(1 - r^n)}{1 - r}$ to calculate these sums efficiently. Geometric series appear in finance: when calculating mortgage payments, the total amount paid is a geometric series with the interest rate as the ratio. Each payment accumulates interest, creating the geometric pattern. The formula lets you calculate loan payoffs, investment growth with regular contributions, and other real-world money situations where compounding occurs! For the series $10 + 10(0.8) + 10(0.8)^2 + 10(0.8)^3 + 10(0.8)^4$ with $a = 10$, $r = 0.8$, $n = 5$: Using the formula $S_n = \frac{a(1 - r^n)}{1 - r}$, we substitute: $S_5 = 10(1 - 0.8^5)/(1 - 0.8) = 10(1 - 0.32768)/0.2 = 10(0.67232)/0.2 = 10 \times 3.3616 = 33.616$. So the sum of the first 5 terms is 33.616. Choice A correctly applies the formula with $a = 10$, $r = 0.8$, $n = 5$ and calculates the sum as 33.616 with accurate arithmetic. Choice B calculates $r^n$ incorrectly: $0.8^4 = 0.4096$ instead of $0.8^5 = 0.32768$; with geometric series, $r^n$ can get large quickly ($2^{10} = 1024$!), so calculator use is often needed. Double-check power calculations! For financial applications like mortgages: the payment $P$, interest rate $r$ (per period), and number of periods $n$ plug into formulas built from geometric series. You might see $S = P((1+r)^n - 1)/r$ or similar—these come from rearranging the geometric series formula! Understanding the underlying geometric series helps you understand why mortgage formulas look the way they do.

Question 20

1. $96$
2. $189$
3. $90$
4. $93$ (correct answer)

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. Let's verify the formula works for a simple example: series 2 + 6 + 18 + 54 with a = 2, r = 3, n = 4. Formula: S_4 = 2(1 - 3^4)/(1 - 3) = 2(1 - 81)/(-2) = 2(-80)/(-2) = 80. Manual addition: 2 + 6 + 18 + 54 = 80 ✓. The formula gives the same answer as adding manually, but it's much faster for large n! For the series 3 + 6 + 12 + 24 + 48 with a = 3, r = 2, n = 5: Using the formula S_5 = 3(1 - 2^5)/(1 - 2) = 3(1 - 32)/(-1) = 3(-31)/(-1) = 3(31) = 93. So the sum of the 5 terms is 93. Choice B correctly applies the formula with a = 3, r = 2, n = 5 and calculates the sum as 93 with accurate arithmetic. Choice D has a sign error: when r > 1, we have negative numerator and negative denominator: (1 - r^n) is negative (1 - big number), and (1 - r) is negative (1 - bigger than 1), so negative/negative = positive. This choice perhaps doubled or miscounted without signs. Track signs carefully! The three-step geometric series strategy: (1) Identify a, r, n: first term, common ratio (divide consecutive terms to find it), number of terms. (2) Check r ≠ 1 (if r = 1, sum = na instead). (3) Substitute into S_n = a(1 - r^n)/(1 - r) and calculate carefully, watching signs: when r > 1, both (1 - r^n) and (1 - r) are negative; when 0 < r < 1, both are positive. The formula handles all cases!