Constructing Linear and Exponential Functions
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Algebra › Constructing Linear and Exponential Functions
If $f$ is linear, $f(0)=8$, and $f(4)=20$, what is $f(x)$? Write in the form $f(x)=mx+b$.
$f(x)=5x+8$
$f(x)=3x+8$
$f(x)=3x+4$
$f(x)=4x+8$
Explanation
This question tests your ability to construct linear functions from given information like points. To construct a linear function from two points, we find the slope using m = (y₂ - y₁)/(x₂ - x₁), then find the y-intercept using one of the points: substitute the point and slope into y = mx + b and solve for b. Once you have m and b, you've got your function y = mx + b! Here, the points are like (0,8) and (4,20): m=(20-8)/(4-0)=12/4=3, and b=8 (given at x=0), so f(x)=3x+8. Choice B correctly constructs the linear function by finding the slope as 3 and using the given intercept 8, giving the function f(x)=3x+8. Perfect! If you misread f(4) or calculate slope wrong, you might pick another, but always use the formula step-by-step. The two-point linear function recipe: (1) Find slope: m = (y₂ - y₁)/(x₂ - x₁) using your two points, (2) Find y-intercept: pick either point, substitute x, y, and m into y = mx + b, solve for b, (3) Write it: y = [your m]x + [your b]. To check, verify both original points work in your function!
A bacteria culture starts with 200 bacteria and grows by 10% each hour. Write a function $P(t)$ for the population after $t$ hours. Write in the form $P(t)=a(1+r)^t$.
$P(t)=200(0.90)^t$
$P(t)=200(1.10)^t$
$P(t)=200(1.10t)$
$P(t)=210(1.10)^t$
Explanation
This question tests your ability to construct exponential functions from given information like descriptions. From context descriptions, listen for the clues: 'per,' 'each,' or 'constant rate' suggest linear (use that rate as slope), while 'percent growth,' 'doubles,' or 'halves' suggest exponential (convert to growth/decay factor). 'Starting with' or 'initial' tells you the y-intercept (linear) or initial value (exponential). The culture 'starts with 200' (initial a=200) and 'grows by 10% each hour' (growth factor 1+0.10=1.10), so $P(t)=200(1.10)^t$. Choice A correctly constructs the exponential function by using the starting amount as a=200 and converting the percent growth to the factor 1.10, giving the function $P(t)=200(1.10)^t$. Perfect! A common mix-up is using addition instead of multiplication for growth, but 'percent' signals exponential—keep practicing to spot it! Context tip: if the problem says 'starts at [value],' that's your y-intercept (linear) or initial value (exponential). If it says 'increases by [number] each time,' that's slope (linear). If it says 'increases by [percent] each time' or 'multiplies by [number],' that's exponential with that as your rate or factor. The language tells you exactly what you need!
An exponential function passes through the points $(0, 2)$ and $(3, 54)$. What is the function in the form $y = a \cdot b^x$?
$y = 2 \cdot 9^x$
$y = 54 \cdot \left(\frac{1}{3}\right)^x$
$y = 2 \cdot(27)^x$
$y = 2 \cdot 3^x$
Explanation
This question tests your ability to construct exponential functions from given information like points. For exponential functions, we need the initial value a (the y-value when x = 0) and the growth factor b. We're given (0, 2), so a = 2. To find b, we use the other point (3, 54). Since y = a · $b^x$, we have 54 = 2 · b³. Solving: b³ = 54/2 = 27, so b = ∛27 = 3. Therefore, y = 2 · $3^x$. Choice A correctly constructs y = 2 · $3^x$ by using the y-intercept point for a = 2 and solving for b = 3 from the second point. Perfect! Choice B with b = 9 would give 2 · 9³ = 2 · 729 = 1458 when x = 3, not 54 - always verify your second point! For exponential from two points: (1) If one point has x = 0, that y-value is your a, (2) Use the other point to find b: substitute into y = a · $b^x$ and solve, (3) Write your function. Check by verifying both points work!
From the coordinate plane, a line passes through the points $(-1, 4)$ and $(3, -4)$. What is the equation of the line? Write in the form $y=mx+b$.
$y=-2x+6$
$y=-2x+2$
$y=2x+2$
$y=-x+3$
Explanation
This question tests your ability to construct linear functions from given information like points. To construct a linear function from two points, we find the slope using m = (y₂ - y₁)/(x₂ - x₁), then find the y-intercept using one of the points: substitute the point and slope into y = mx + b and solve for b. Once you have m and b, you've got your function y = mx + b! For points (-1,4) and (3,-4): m=(-4-4)/(3-(-1))=-8/4=-2; using (-1,4): 4=-2*(-1)+b, so b=2, giving y=-2x+2. Choice B correctly constructs the linear function by finding the slope as -2 and the intercept as 2, giving the function y=-2x+2. Perfect! It's common to slip on negative signs or point selection, but checking both points in the final equation ensures it's spot on. The two-point linear function recipe: (1) Find slope: m = (y₂ - y₁)/(x₂ - x₁) using your two points, (2) Find y-intercept: pick either point, substitute x, y, and m into y = mx + b, solve for b, (3) Write it: y = [your m]x + [your b]. To check, verify both original points work in your function!
A linear function passes through the points $(2, 7)$ and $(6, 19)$. What is the equation of the function in the form $y = mx + b$?
$y = 4x - 1$
$y = 3x + 1$
$y = 2x + 3$
$y = 3x + 3$
Explanation
This question tests your ability to construct linear functions from given information like points. To construct a linear function from two points, we find the slope using m = (y₂ - y₁)/(x₂ - x₁), then find the y-intercept using one of the points: substitute the point and slope into y = mx + b and solve for b. Once you have m and b, you've got your function y = mx + b! Let's find the slope: m = (19 - 7)/(6 - 2) = 12/4 = 3. Now using point (2, 7): 7 = 3(2) + b, so 7 = 6 + b, which gives us b = 1. Choice B correctly constructs y = 3x + 1 by finding slope = 3 and y-intercept = 1, giving the function y = 3x + 1. Perfect! Choice A incorrectly calculates the slope as 2 instead of 3 - remember to carefully compute the rise over run! The two-point linear function recipe: (1) Find slope: m = (y₂ - y₁)/(x₂ - x₁) using your two points, (2) Find y-intercept: pick either point, substitute x, y, and m into y = mx + b, solve for b, (3) Write it: y = [your m]x + [your b]. To check, verify both original points work in your function!
A gym charges a $\$25$ sign-up fee plus $$10$ per month. Let $m$ be the number of months and $C(m)$ be the total cost (in dollars). Write the linear function $C(m)$.
$C(m) = 35m$
$C(m) = 25m + 10$
$C(m) = 25 + 10^m$
$C(m) = 10m + 25$
Explanation
This question tests your ability to construct linear functions from given information like descriptions. From context descriptions, listen for the clues: 'per,' 'each,' or 'constant rate' suggest linear (use that rate as slope), while 'percent growth,' 'doubles,' or 'halves' suggest exponential. 'Starting with' or 'initial' tells you the y-intercept (linear) or initial value (exponential). The gym charges a $25 sign-up fee (this is our y-intercept, the cost when m = 0) plus $10 per month (this is our slope, the rate of change). So C(m) = 10m + 25. Choice B correctly constructs C(m) = 10m + 25 by identifying the monthly rate as the slope (10) and the sign-up fee as the y-intercept (25). Perfect! Choice A incorrectly swaps these values - remember, the 'per month' rate is what multiplies the variable, while the one-time fee is the constant term. Context tip: if the problem says 'starts at [value],' that's your y-intercept. If it says 'increases by [number] each time,' that's slope. The language tells you exactly what you need!
A population of bacteria starts at 200 and grows by 10% each hour. Let $t$ be the number of hours. Which function models the population $P(t)$?
$P(t) = 200(0.9)^t$
$P(t) = 210^t$
$P(t) = 200 + 0.10t$
$P(t) = 200(1.1)^t$
Explanation
This question tests your ability to construct exponential functions from given information like descriptions. From context descriptions, listen for the clues: 'percent growth' suggests exponential (convert to growth factor). 'Starting with' tells you the initial value. The bacteria start at 200 (initial value a = 200) and grow by 10% each hour. Growing by 10% means multiplying by 1.10 (100% + 10% = 110% = 1.10), so b = 1.1. The function is P(t) = $200(1.1)^t$. Let's verify: P(0) = $200(1.1)^0$ = 200 ✓; P(1) = $200(1.1)^1$ = 220, which is 200 + 10% of 200 ✓. Choice C correctly constructs P(t) = $200(1.1)^t$ by identifying initial value = 200 and growth factor = 1.1, giving the function P(t) = $200(1.1)^t$. Perfect! Choice A uses 0.9 (which would be 10% decay, not growth), B is linear, and D has no initial value coefficient. Context tip: if the problem says 'increases by [percent] each time,' that's exponential with growth factor = 1 + (percent as decimal). For 10% growth, b = 1 + 0.10 = 1.10. The language tells you exactly what you need!
A gym charges a $\$25 sign-up fee plus $\$15 per month. Write a function $C(m)$ for the total cost after $m$ months. Write in the form $C(m)=am+b$.
$C(m)=15m+25$
$C(m)=25m+15$
$C(m)=40m$
$C(m)=25+15^m$
Explanation
This question tests your ability to construct linear functions from given information like descriptions. From context descriptions, listen for the clues: 'per,' 'each,' or 'constant rate' suggest linear (use that rate as slope), while 'percent growth,' 'doubles,' or 'halves' suggest exponential (convert to growth/decay factor). 'Starting with' or 'initial' tells you the y-intercept (linear) or initial value (exponential). Here, the $15 per month is the constant rate (slope a=15), and the $25 sign-up is the initial fixed cost (intercept b=25), so C(m)=15m+25. Choice B correctly constructs the linear function by using the monthly rate as the slope 15 and the sign-up as the intercept 25, giving the function C(m)=15m+25. Perfect! It's easy to swap the coefficients if you're not careful, but remembering 'per month' multiplies with m helps avoid that. Context tip: if the problem says 'starts at [value],' that's your y-intercept (linear) or initial value (exponential). If it says 'increases by [number] each time,' that's slope (linear). If it says 'increases by [percent] each time' or 'multiplies by [number],' that's exponential with that as your rate or factor. The language tells you exactly what you need!
A bacteria culture starts with 200 bacteria and increases by 10% each hour. Write a function $P(t)$ for the population after $t$ hours.
$P(t)=210t$
$P(t)=10(200)^t$
$P(t)=200(0.90)^t$
$P(t)=200(1.10)^t$
Explanation
This question tests your ability to construct exponential functions from given information like descriptions. From context descriptions, listen for the clues: 'per,' 'each,' or 'constant rate' suggest linear (use that rate as slope), while 'percent growth,' 'doubles,' or 'halves' suggest exponential (convert to growth/decay factor). 'Starting with' or 'initial' tells you the y-intercept (linear) or initial value (exponential). Starts with 200 (initial a=200), increases by 10% each hour means growth factor b=1+0.10=1.10, so $P(t)=200(1.10)^t$. Choice A correctly constructs the exponential by using initial 200 and factor 1.10, giving $P(t)=200(1.10)^t$. Perfect! A mistake could be using decay like 0.90, but since it's increase, add to 1 for the factor. Context tip: if the problem says 'starts at [value],' that's your y-intercept (linear) or initial value (exponential). If it says 'increases by [number] each time,' that's slope (linear). If it says 'increases by [percent] each time' or 'multiplies by [number],' that's exponential with that as your rate or factor. The language tells you exactly what you need! For exponential from a table: (1) Find the initial value by looking at x = 0 (if in table) or work backward using the pattern, (2) Find the common ratio by dividing consecutive y-values: y₂/y₁ (should be the same for all consecutive pairs), (3) Write y = $a·b^x$ with your values. Example: if y goes 5, 10, 20 as x goes 0, 1, 2, then a = 5, b = 10/5 = 2, so y = $5·2^x$!
If $f$ is linear, $f(0)=4$, and $f(3)=13$, what is $f(x)$? Write in the form $f(x)=mx+b$.
$f(x)=3x-4$
$f(x)=9x+4$
$f(x)=3x+4$
$f(x)=4x+3$
Explanation
This question tests your ability to construct linear functions from given information like points. To construct a linear function from two points, we find the slope using m = (y₂ - y₁)/(x₂ - x₁), then find the y-intercept using one of the points: substitute the point and slope into y = mx + b and solve for b. Once you have m and b, you've got your function y = mx + b! Here, points are like (0,4) and (3,13): m=(13-4)/(3-0)=9/3=3, and b=4 given at x=0, so f(x)=3x+4. Choice B correctly constructs the function by finding slope 3 and intercept 4, giving f(x)=3x+4. Perfect! Swapping numbers might give 4x+3, but remember slope is change in y over change in x. The two-point linear function recipe: (1) Find slope: m = (y₂ - y₁)/(x₂ - x₁) using your two points, (2) Find y-intercept: pick either point, substitute x, y, and m into y = mx + b, solve for b, (3) Write it: y = [your m]x + [your b]. To check, verify both original points work in your function!