Find and Write an Inverse Function
Help Questions
Algebra 2 › Find and Write an Inverse Function
If $f(x)=(x+2)^3$, solve $f(x)=27$ and express the solution as $f^{-1}(27)$.
$f^{-1}(27)=3$
$f^{-1}(27)=-1$
$f^{-1}(27)=1$
$f^{-1}(27)=5$
Explanation
This question tests your ability to find inverse functions—functions that undo what the original function does, reversing the input-output relationship. An inverse function f⁻¹(x) reverses f(x): if f takes a to b, then f⁻¹ takes b back to a. To find the inverse algebraically, use the swap-and-solve method: (1) write y = f(x), (2) swap x and y (this reverses the input-output roles), (3) solve for y, (4) the result is y = f⁻¹(x). For example, if f(x) = 2x + 3, write y = 2x + 3, swap to x = 2y + 3, solve to get y = (x - 3)/2, so f⁻¹(x) = (x - 3)/2. This inverse undoes the 'multiply by 2 then add 3' by doing 'subtract 3 then divide by 2'! To find $f^{-1}$(27), solve $(x+2)^3$=27, take cube root x+2=3, then x=1. Choice B correctly finds $f^{-1}$(27)=1 by solving properly. Choice A fails by forgetting to subtract the 2 after taking the cube root; always reverse all operations in the function. Inverse thinking: ask yourself 'what operations does f do, and in what order?' then reverse the order and undo each operation. If f(x) = $(x+2)^3$ does 'add 2, then cube,' the inverse should do 'cube root, then subtract 2' to get back to x.
Determine the inverse function for $f(x)=x^3+5$.
$f^{-1}(x)=\sqrt[3]{x-5}$
$f^{-1}(x)=\dfrac{1}{x^3+5}$
$f^{-1}(x)=\sqrt[3]{x}+5$
$f^{-1}(x)=\sqrt[3]{5-x}$
Explanation
This question tests your ability to find inverse functions—functions that undo what the original function does, reversing the input-output relationship. An inverse function f⁻¹(x) reverses f(x): if f takes a to b, then f⁻¹ takes b back to a. To find the inverse algebraically, use the swap-and-solve method: (1) write y = f(x), (2) swap x and y (this reverses the input-output roles), (3) solve for y, (4) the result is y = f⁻¹(x). For example, if f(x) = 2x + 3, write y = 2x + 3, swap to x = 2y + 3, solve to get y = (x - 3)/2, so f⁻¹(x) = (x - 3)/2. This inverse undoes the 'multiply by 2 then add 3' by doing 'subtract 3 then divide by 2'! For $f(x)=x^3$+5, set $y=x^3$+5, swap $x=y^3$+5, subtract 5: $x-5=y^3$, take cube root y=\sqrt[3]{x-5}. Choice B correctly finds f⁻¹(x)=\sqrt[3]{x-5} by swapping and solving properly. Choice D confuses inverse with reciprocal; remember to isolate the variable correctly after swapping. Inverse thinking: ask yourself 'what operations does f do, and in what order?' then reverse the order and undo each operation. If f(x) = $x^3$ + 5 does 'cube, then add 5,' the inverse should do 'subtract 5, then cube root': \sqrt[3]{x-5}. This intuitive approach helps you predict what the inverse should be before computing it algebraically!
Solve $f(x)=16$ where $f(x)=2x^3$, and express your answer as $f^{-1}(16)$.
$f^{-1}(16)=\dfrac{1}{16}$
$f^{-1}(16)=\sqrt[3]{8}=2$
$f^{-1}(16)=\sqrt[3]{\dfrac{16}{2}}=2$
$f^{-1}(16)=\sqrt[3]{\dfrac{2}{16}}$
Explanation
This question tests your ability to find inverse functions—functions that undo what the original function does, reversing the input-output relationship. The inverse notation $f^{-1}(x)$ does NOT mean $1/f(x)$ (that would be the reciprocal)—the superscript -1 indicates inverse function, not exponentiation. This is a very common source of confusion! The inverse undoes the function's operation, while the reciprocal is just division. For $f(x) = 2x$, the inverse is $f^{-1}(x) = x/2$ (undoes multiplying by 2), but the reciprocal is $1/(2x)$ (completely different!). To find $f^{-1}(16)$, solve $2x^3=16$, divide by 2: $x^3=8$, take cube root $x=2$, or equivalently $\sqrt[3]{16/2}=\sqrt[3]{8}=2$. Choice B correctly finds $f^{-1}(16)=\sqrt[3]{16/2}=2$ by solving properly. Choice C reverses the fraction inside the root, a common solving mistake; remember to divide by 2 before rooting. Inverse thinking: ask yourself 'what operations does $f$ do, and in what order?' then reverse the order and undo each operation. If $f(x) = 2x^3$ does 'cube, then multiply by 2,' the inverse should do 'divide by 2, then cube root': $\sqrt[3]{x/2}$. This intuitive approach helps you predict what the inverse should be before computing it algebraically!
Write an expression for $f^{-1}(x)$ given $f(x)=\dfrac{x-4}{3}$.
$f^{-1}(x)=\dfrac{x+4}{3}$
$f^{-1}(x)=3x-4$
$f^{-1}(x)=3x+4$
$f^{-1}(x)=\dfrac{3}{x-4}$
Explanation
This question tests your ability to find inverse functions—functions that undo what the original function does, reversing the input-output relationship. The inverse notation $f^{-1}(x)$ does NOT mean $1/f(x)$ (that would be the reciprocal)—the superscript -1 indicates inverse function, not exponentiation. This is a very common source of confusion! The inverse undoes the function's operation, while the reciprocal is just division. For $f(x) = 2x$, the inverse is $f^{-1}(x) = \dfrac{x}{2}$ (undoes multiplying by 2), but the reciprocal is $1/(2x)$ (completely different!). For $f(x)=\dfrac{x-4}{3}$, set $y=\dfrac{x-4}{3}$, swap $x=\dfrac{y-4}{3}$, multiply by 3: $3x=y-4$, add 4: $y=3x+4$. Choice C correctly finds $f^{-1}(x)=3x+4$ by swapping and solving properly. Choice D might stem from reciprocal confusion or incorrect solving; always verify by composition. The swap-and-solve recipe: (1) Replace f(x) with y to get $y = $ [formula], (2) Swap every x with y and every y with x: $x = $ [formula with y], (3) Solve this equation for y using algebra (isolate y), (4) The expression for y is your $f^{-1}(x)$. Verify your answer: compute $f(f^{-1}(x))$ and it should simplify to just x. If it doesn't, recheck your algebra! This verification catches most errors.
Find and verify the inverse of $f(x)=\dfrac{2x+3}{5}$ by checking that $f\big(f^{-1}(x)\big)=x$.
$f^{-1}(x)=\dfrac{5}{2x+3}$
$f^{-1}(x)=\dfrac{5x-3}{2}$
$f^{-1}(x)=\dfrac{2x+3}{5}$
$f^{-1}(x)=\dfrac{5x+3}{2}$
Explanation
This question tests your ability to find inverse functions—functions that undo what the original function does, reversing the input-output relationship. The inverse notation f⁻¹(x) does NOT mean 1/f(x) (that would be the reciprocal)—the superscript -1 indicates inverse function, not exponentiation. This is a very common source of confusion! The inverse undoes the function's operation, while the reciprocal is just division. For f(x) = 2x, the inverse is f⁻¹(x) = x/2 (undoes multiplying by 2), but the reciprocal is 1/(2x) (completely different!). For f(x)=(2x+3)/5, set y=(2x+3)/5, swap x=(2y+3)/5, multiply by 5: 5x=2y+3, subtract 3: 5x-3=2y, divide by 2: y=(5x-3)/2. Choice B correctly finds f⁻¹(x)=(5x-3)/2 by swapping and solving properly. Choice D confuses inverse with reciprocal; use swap-and-solve and verify by composing $f(f^{-1}$(x))=x. The swap-and-solve recipe: (1) Replace f(x) with y to get y = [formula], (2) Swap every x with y and every y with x: x = [formula with y], (3) Solve this equation for y using algebra (isolate y), (4) The expression for y is your f⁻¹(x). Verify your answer: compute f(f⁻¹(x)) and it should simplify to just x. If it doesn't, recheck your algebra! This verification catches most errors.
Write an expression for $f^{-1}(x)$ given $f(x)=3x-7$.
$f^{-1}(x)=\dfrac{3}{x-7}$
$f^{-1}(x)=\dfrac{x+7}{3}$
$f^{-1}(x)=\dfrac{x-7}{3}$
$f^{-1}(x)=3x+7$
Explanation
This question tests your ability to find inverse functions—functions that undo what the original function does, reversing the input-output relationship. The inverse notation f⁻¹(x) does NOT mean 1/f(x) (that would be the reciprocal)—the superscript -1 indicates inverse function, not exponentiation. This is a very common source of confusion! The inverse undoes the function's operation, while the reciprocal is just division. For f(x) = 2x, the inverse is f⁻¹(x) = x/2 (undoes multiplying by 2), but the reciprocal is 1/(2x) (completely different!). For f(x)=3x-7, set y=3x-7, swap to x=3y-7, add 7 to get x+7=3y, divide by 3: y=(x+7)/3. Choice A correctly finds f⁻¹(x)=(x+7)/3 by swapping and solving properly. Choice D confuses inverse with reciprocal; remember, reciprocal would be 1/(3x-7), but inverse requires swap-and-solve. The swap-and-solve recipe: (1) Replace f(x) with y to get y = [formula], (2) Swap every x with y and every y with x: x = [formula with y], (3) Solve this equation for y using algebra (isolate y), (4) The expression for y is your f⁻¹(x). Verify your answer: compute f(f⁻¹(x)) and it should simplify to just x. If it doesn't, recheck your algebra! This verification catches most errors.
What is $f^{-1}(x)$ if $f(x)=\dfrac{x+1}{x-1}$, where $x\ne 1$? (Note: $f^{-1}$ means inverse function, not reciprocal.)
$f^{-1}(x)=\dfrac{x+1}{x-1}$ (same as $f$)
$f^{-1}(x)=\dfrac{x-1}{x+1}$
$f^{-1}(x)=\dfrac{x+1}{x-1}$
$f^{-1}(x)=\dfrac{1}{\frac{x+1}{x-1}}$
Explanation
This question tests your ability to find inverse functions—functions that undo what the original function does, reversing the input-output relationship. An inverse function $f^{-1}(x)$ reverses $f(x)$: if $f$ takes a to b, then $f^{-1}$ takes b back to a. To find the inverse algebraically, use the swap-and-solve method: (1) write $y = f(x)$, (2) swap x and y (this reverses the input-output roles), (3) solve for y, (4) the result is $y = f^{-1}(x)$. For example, if $f(x) = 2x + 3$, write $y = 2x + 3$, swap to $x = 2y + 3$, solve to get $y = (x - 3)/2$, so $f^{-1}(x) = (x - 3)/2$. This inverse undoes the 'multiply by 2 then add 3' by doing 'subtract 3 then divide by 2'! For $f(x)=\dfrac{x+1}{x-1}$, set $y=\dfrac{x+1}{x-1}$, swap to $x=\dfrac{y+1}{y-1}$, then solve: $x(y-1)=y+1$, $xy-x=y+1$, $xy-y=x+1$, $y(x-1)=x+1$, $y=\dfrac{x+1}{x-1}$. Choice B correctly finds $f^{-1}(x)=\dfrac{x+1}{x-1}$ (same as $f$) by swapping and solving properly. Choice A might result from incorrectly swapping signs during algebra; double-check by verifying $f(f^{-1}(x))=x$ to catch such errors. Inverse thinking: ask yourself 'what operations does $f$ do, and in what order?' then reverse the order and undo each operation. If $f(x) = 2x + 3$ does 'multiply by 2, then add 3,' the inverse should do 'subtract 3, then divide by 2': $(x - 3)/2$. This intuitive approach helps you predict what the inverse should be before computing it algebraically!
Solve $f(x)=16$ where $f(x)=2x^3$, and express your answer as $f^{-1}(16)$.
$f^{-1}(16)=\sqrt[3]{8}$
$f^{-1}(16)=\sqrt[3]{32}$
$f^{-1}(16)=\sqrt[3]{\frac{16}{2}}$
$f^{-1}(16)=\dfrac{1}{16}$
Explanation
This question tests your ability to find inverse functions—functions that undo what the original function does, reversing the input-output relationship. An inverse function f⁻¹(x) reverses f(x): if f takes a to b, then f⁻¹ takes b back to a. To find the inverse algebraically, use the swap-and-solve method: (1) write y = f(x), (2) swap x and y (this reverses the input-output roles), (3) solve for y, (4) the result is y = f⁻¹(x). For example, if f(x) = 2x + 3, write y = 2x + 3, swap to x = 2y + 3, solve to get y = (x - 3)/2, so f⁻¹(x) = (x - 3)/2. This inverse undoes the 'multiply by 2 then add 3' by doing 'subtract 3 then divide by 2'! For f(x) = 2x³, to find $f^{-1}$(16) means solving 2x³ = 16 for x, so x³ = 16/2 = 8, x = sqrt[3]{8} = 2, which is expressed as sqrt[3]{16/2}; alternatively, the inverse is y = 2x³, swap x = 2y³, y = sqrt[3]{x/2}, so $f^{-1}$(16) = sqrt[3]{16/2}. Choice B correctly finds $f^{-1}$(16) = sqrt[3]{16/2} by properly solving or evaluating the inverse. Choice D fails by taking the reciprocal incorrectly instead of reversing the operations. Inverse thinking: ask yourself 'what operations does f do, and in what order?' then reverse the order and undo each operation. If f(x) = 2x + 3 does 'multiply by 2, then add 3,' the inverse should do 'subtract 3, then divide by 2': (x - 3)/2. If f(x) = x³ does 'cube,' the inverse should do 'cube root': ∛x. This intuitive approach helps you predict what the inverse should be before computing it algebraically!
Determine the inverse function for $f(x)=\sqrt3{x-8}$.
$f^{-1}(x)=\sqrt[3]{x}+8$
$f^{-1}(x)=x^3-8$
$f^{-1}(x)=\dfrac{1}{\sqrt[3]{x-8}}$
$f^{-1}(x)=x^3+8$
Explanation
This question tests your ability to find inverse functions—functions that undo what the original function does, reversing the input-output relationship. The inverse notation $f^{-1}(x)$ does NOT mean $1/f(x)$ (that would be the reciprocal)—the superscript -1 indicates inverse function, not exponentiation. This is a very common source of confusion! The inverse undoes the function's operation, while the reciprocal is just division. For $f(x) = 2x$, the inverse is $f^{-1}(x) = x/2$ (undoes multiplying by 2), but the reciprocal is $1/(2x)$ (completely different!). To find the inverse of $f(x) = \sqrt[3]{x - 8}$, write $y = \sqrt[3]{x - 8}$, swap x and y to get $x = \sqrt[3]{y - 8}$, solve for y by cubing both sides: $x^3 = y - 8$, then add 8: $y = x^3 + 8$. Choice B correctly finds $f^{-1}(x) = x^3 + 8$ by swapping and solving properly. Choice D fails by incorrectly incorporating a reciprocal and mishandling the cube root. The swap-and-solve recipe: (1) Replace f(x) with y to get $y = \sqrt[3]{x - 8}$, (2) Swap every x with y and every y with x: $x = \sqrt[3]{y - 8}$, (3) Solve this equation for y using algebra (isolate y), (4) The expression for y is your $f^{-1}(x)$. Verify your answer: compute $f(f^{-1}(x))$ and it should simplify to just x. If it doesn't, recheck your algebra! This verification catches most errors.
Find the inverse function $f^{-1}(x)$ for $f(x)=\dfrac{x-4}{3}$.
$f^{-1}(x)=\dfrac{3}{x-4}$
$f^{-1}(x)=3x+4$
$f^{-1}(x)=3x-4$
$f^{-1}(x)=\dfrac{x-4}{3}$
Explanation
This question tests your ability to find inverse functions—functions that undo what the original function does, reversing the input-output relationship. The inverse notation f⁻¹(x) does NOT mean 1/f(x) (that would be the reciprocal)—the superscript -1 indicates inverse function, not exponentiation. This is a very common source of confusion! The inverse undoes the function's operation, while the reciprocal is just division. For f(x) = 2x, the inverse is f⁻¹(x) = x/2 (undoes multiplying by 2), but the reciprocal is 1/(2x) (completely different!). To find the inverse of f(x) = (x - 4)/3, write y = (x - 4)/3, swap x and y to get x = (y - 4)/3, solve for y by multiplying both sides by 3: 3x = y - 4, then add 4: y = 3x + 4. Choice C correctly finds f⁻¹(x) = 3x + 4 by swapping and solving properly. Choice D fails by incorrectly taking a reciprocal form, likely from a solving mistake. The swap-and-solve recipe: (1) Replace f(x) with y to get y = (x - 4)/3, (2) Swap every x with y and every y with x: x = (y - 4)/3, (3) Solve this equation for y using algebra (isolate y), (4) The expression for y is your f⁻¹(x). Verify your answer: compute f(f⁻¹(x)) and it should simplify to just x. If it doesn't, recheck your algebra! This verification catches most errors.