Algebraic Properties of Limits

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AP Calculus BC › Algebraic Properties of Limits

Questions 1 - 10
1

A concentration is $k(x)=\dfrac{x^2-4x+1}{x}$; find $\lim_{x\to 5} k(x)$.

$25-20+\dfrac{1}{5}$

$\dfrac{(5-4)^2+1}{5}$

$\dfrac{5^2-4+1}{5}$

$\dfrac{25-4(5)+1}{0}$

$\dfrac{25-20+1}{5}$

Explanation

This problem requires using the limit laws for rational functions. The denominator at x=5 is 5 ≠0, so direct substitution is applicable. The limit is $(5^2$ -4*5 +1)/5 = (25-20+1)/5 =6/5. The choice A is this expression. A tempting distractor is D d$\frac{25-4(5)+1}{0}$, perhaps from incorrectly using 0 in denominator. A transferable strategy is to substitute the value into both numerator and denominator separately and then divide.

2

As $t$ approaches $2$ seconds, the sensor output is $S(t)=3t^2-5t+4$; find $\lim_{t\to 2} S(t)$.

$2$

$6$

$3(2)^2-5+4$

$3(2)^2-5(2)+4$

$6(2)^2-5(2)+4$

Explanation

This problem requires using the limit laws, specifically the direct substitution property for polynomials. Since S(t) is a polynomial, it is continuous, and the limit as t approaches 2 is S(2) = $3(2)^2$ -5(2) +4. This expression simplifies to 12 -10 +4 =6. The choice D is this exact expression for direct substitution. A tempting distractor is C, which uses 6 as the coefficient instead of 3, perhaps from misreading $3t^2$ as $6t^2$, leading to an incorrect calculation. A transferable strategy is to identify if the function is continuous at the point and substitute directly for the limit.

3

A signal is modeled by $f(x)=\dfrac{5-2x}{x^2+1}$. Find $\lim_{x\to 0} f(x)$.

$5$

$\dfrac{5}{2}$

$0$

$-5$

$\dfrac{3}{1}$

Explanation

This problem requires using limit laws on a rational function where the denominator is non-zero at the limit point. Since $0^2+1 = 1 \neq 0$, we can apply direct substitution: $\lim_{x\to 0} \frac{5-2x}{x^2+1} = \frac{5-2(0)}{0^2+1} = \frac{5-0}{0+1} = \frac{5}{1} = 5$. The limit laws allow us to evaluate the numerator and denominator separately when the denominator limit is non-zero. A potential error would be to think the limit is 0 because $x$ approaches 0, forgetting that we need to evaluate the entire expression. For rational functions, always substitute the limit value into both numerator and denominator, checking that the denominator remains non-zero.

4

Given $R(x)=\dfrac{(x+1)(x-3)}{x-3}$ for $x\ne 3$, evaluate $\lim_{x\to 3} R(x)$.

$\dfrac{(3+1)(3-3)}{3}$

$\dfrac{(3+1)(3)}{3-3}$

$\dfrac{(3)(3-3)}{3-3}$

$\dfrac{(3+1)(3-3)}{3-3}$

$3+1$

Explanation

This problem requires using the algebraic properties of limits, including simplifying rational expressions before taking the limit. The function R(x) = (x+1)(x-3)/(x-3) simplifies to x+1 for x≠3. Therefore, the limit as x approaches 3 is 3+1=4. The choice B is this value. A tempting distractor is A, which plugs into the original to get 0/0, an indeterminate form that doesn't give the limit. A transferable strategy is to factor and cancel common factors in rational functions when direct substitution gives 0/0.

5

A growth function is $P(x)=7-\dfrac{3}{x}$. Determine $\lim_{x\to 6} P(x)$.

$\dfrac{13}{2}$

$\dfrac{3}{7}$

$\dfrac{11}{2}$

$\dfrac{4}{3}$

$\dfrac{7}{2}$

Explanation

This problem uses limit laws to evaluate a function involving subtraction of a rational term. Since $x=6$ makes the denominator non-zero, we can use direct substitution: $\lim_{x\to 6} \left(7-\frac{3}{x}\right) = 7-\frac{3}{6} = 7-\frac{1}{2} = \frac{14}{2}-\frac{1}{2} = \frac{13}{2}$. The limit laws tell us we can evaluate the limit of a difference by taking the difference of the individual limits. A potential mistake would be to compute $7-3 = 4$ and then divide by 6, getting $\frac{4}{6} = \frac{2}{3}$, by misapplying the order of operations. For expressions with multiple operations, always follow the correct order: division before subtraction.

6

A particle’s position is $s(t)=\dfrac{t^2+1}{t+1}$. Determine $\lim_{t\to 3} s(t)$.

$\dfrac{10}{3}$

$\dfrac{3}{4}$

$\dfrac{4}{5}$

$\dfrac{10}{4}$

$\dfrac{5}{2}$

Explanation

This problem uses limit laws to evaluate a rational function where direct substitution is valid. Since the denominator $t+1$ is non-zero when $t=3$, we can substitute directly: $\lim_{t\to 3} \frac{t^2+1}{t+1} = \frac{3^2+1}{3+1} = \frac{9+1}{4} = \frac{10}{4} = \frac{5}{2}$. The limit laws allow us to evaluate the numerator and denominator limits separately when the denominator limit is non-zero. A common error would be to simplify incorrectly, perhaps getting $\frac{10}{3}$ by mistakenly using $t$ instead of $t+1$ in the denominator. For rational functions, always verify the denominator is non-zero at the limit point before applying direct substitution.

7

If $m(x)=\dfrac{3x-6}{x+5}+2$, find $\lim_{x\to 1} m(x)$.

$\dfrac{3(1)-6}{1}+5+2$

$\dfrac{3(1-6)}{1+5}+2$

$\dfrac{3-6}{1}+5+2$

$\dfrac{3(1)-6}{1+5+2}$

$\dfrac{3(1)-6}{1+5}+2$

Explanation

This problem requires using the limit laws for sums and quotients. The function is continuous at x=1 since denominator 1+5=6 ≠0. The limit is [3(1)-6]/(1+5) +2 = (-3)/6 +2 = -0.5 +2=1.5. The choice A is this expression. A tempting distractor is B d$\frac{3(1-6)}{1+5}$+2 = 3*(-5)/6 +2 = -15/6 +2 = -2.5+2=-0.5, from misplacing the parentheses in numerator. A transferable strategy is to apply substitution to each component and combine using the sum law.

8

During a lab, $g(x)=\dfrac{2x^2+3x-5}{x-1}$ is recorded; evaluate $\lim_{x\to 1} (2x^2+3x-5)$.

$4$

$-4$

$0$

$-2$

$5$

Explanation

This problem illustrates the use of limit laws, specifically the sum, difference, and constant multiple rules for limits. The expression $2x^2$ + 3x - 5 is a polynomial, so the limit as x approaches 1 is simply its value at x=1. Substituting x=1 gives $2*(1)^2$ + 3*1 - 5 = 2 + 3 - 5 = 0. This follows from applying the power rule to each term and then using the sum and difference properties of limits. A tempting distractor like -4 might come from confusing it with evaluating g(x), but the question asks only for the numerator, which is defined at x=1. A transferable strategy is to apply direct substitution for polynomials, as they are continuous everywhere.

9

A function is $m(x)=\dfrac{5}{2}x^2-4x+9$; evaluate $\lim_{x\to 0} m(x)$.

$\dfrac{5}{2}$

$9$

$-4$

$0$

$-9$

Explanation

This problem illustrates the use of limit laws, specifically the sum, difference, and constant multiple rules. m(x) is a polynomial, continuous everywhere, so the limit as x approaches 0 is m(0). Substituting x=0 gives (5/2)0 - 40 + 9 = 9. This follows from applying the power rule to each term and combining with sum properties. A tempting distractor like 0 might come from only considering the variable terms, but the constant 9 remains. A transferable strategy is to evaluate polynomials at the limit point directly due to their continuity.

10

A projectile’s height is $h(t)=\dfrac{t^2+3t}{t+3}$; compute $\lim_{t\to 1} h(t)$.

$\dfrac{1^2+3(1)}{3}$

$1$

$\dfrac{1^2+3(1)}{1}+3$

$\dfrac{1^2+3(1)}{1+3}$

$\dfrac{1^2+3}{1+3}$

Explanation

This problem requires using the limit laws for rational functions. Since the denominator at t=1 is 1+3=4 ≠0, direct substitution is valid. The limit is $(1^2$ +3*1)/(1+3) = (1+3)/4 =1. The choice B is this plugged-in expression. A tempting distractor is D $d$\frac{1^2$ +3(1)}{3}$ =4/3, perhaps from misplacing the denominator as 3 alone. A transferable strategy is to evaluate the denominator's limit first to ensure it's not zero before applying direct substitution.

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