Find and Write an Inverse Function
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Algebra 2 › Find and Write an Inverse Function
What is $f^{-1}(x)$ if $f(x)=\dfrac{x}{x+2}$, where $x\ne -2$?
$f^{-1}(x)=\dfrac{x+2}{x}$, where $x\ne 0$
$f^{-1}(x)=\dfrac{1-x}{2x}$, where $x\ne 0$
$f^{-1}(x)=\dfrac{x}{x-2}$, where $x\ne 2$
$f^{-1}(x)=\dfrac{2x}{1-x}$, where $x\ne 1$
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) = x/(x + 2), write y = x/(x + 2), swap to x = y/(y + 2), multiply by (y + 2): x(y + 2) = y, xy + 2x = y, 2x = y - xy, 2x = y(1 - x), y = 2x/(1 - x). Choice B correctly finds f⁻¹(x) = 2x/(1 - x) by swapping and solving properly. Choice A fails by simplifying incorrectly, giving x/(x - 2), perhaps from a sign error in collecting terms—double-check algebra steps! 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!
What function undoes $f(x)=\dfrac{x-4}{3}$? (That is, find $f^{-1}(x)$.)
$f^{-1}(x)=3x-4$
$f^{-1}(x)=\dfrac{3}{x-4}$
$f^{-1}(x)=3x+4$
$f^{-1}(x)=\dfrac{x-4}{3}$
Explanation
This question tests your ability to find inverse functions—functions that undo what the original function does, reversing the input-output relationship. 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 f(x) = (x-4)/3, let's apply swap-and-solve: Start with y = (x-4)/3, swap to get x = (y-4)/3, multiply both sides by 3 to get 3x = y - 4, then add 4 to both sides to get y = 3x + 4, so f⁻¹(x) = 3x + 4. Choice C correctly finds f⁻¹(x) = 3x + 4 by properly swapping and solving—it undoes 'subtract 4 then divide by 3' with 'multiply by 3 then add 4.' Choice A shows a sign error in the final step, while Choice D incorrectly treats this as a reciprocal rather than an inverse function. 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 (multiply by 3, add 4), (4) The expression for y is your f⁻¹(x) = 3x + 4. 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-4)/3 does 'subtract 4, then divide by 3,' the inverse should do 'multiply by 3, then add 4': 3x + 4!
Find and verify the inverse of $f(x)=\dfrac{2x+3}{5}$ by using the idea that $f\big(f^{-1}(x)\big)=x$.
$f^{-1}(x)=\dfrac{5x-3}{2}$
$f^{-1}(x)=\dfrac{2x-3}{5}$
$f^{-1}(x)=\dfrac{5x+3}{2}$
$f^{-1}(x)=\dfrac{5}{2x+3}$
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) = (2x + 3)/5, write y = (2x + 3)/5, swap to x = (2y + 3)/5, multiply by 5 to get 5x = 2y + 3, subtract 3 to get 5x - 3 = 2y, divide by 2: y = (5x - 3)/2. Choice A correctly finds f⁻¹(x) = (5x - 3)/2 by swapping and solving properly. Choice D fails by confusing with reciprocal, giving 5/(2x + 3), but inverses undo operations, not reciprocate. The swap-and-solve recipe: (1) Replace f(x) with y to get y = (2x + 3)/5, (2) Swap every x with y and every y with x: x = (2y + 3)/5, (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 $f(x)=x^3+5$.
$f^{-1}(x)=\sqrt[3]{x-5}$
$f^{-1}(x)=\sqrt[3]{x}+5$
$f^{-1}(x)=\sqrt[3]{x+5}$
$f^{-1}(x)=\dfrac{1}{x^3+5}$
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 the inverse of f(x) = x³ + 5, write y = x³ + 5, swap x and y to get x = y³ + 5, solve for y by subtracting 5: x - 5 = y³, then take the 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 fails by mistakenly taking the reciprocal instead of reversing the operations correctly. 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!
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.
What is $f^{-1}(x)$ if $f(x)=3x-7$?
$f^{-1}(x)=\dfrac{x+7}{3}$
$f^{-1}(x)=\dfrac{3}{x-7}$
$f^{-1}(x)=3x+7$
$f^{-1}(x)=\dfrac{x-7}{3}$
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) = 3x - 7, write y = 3x - 7, swap x and y to get x = 3y - 7, solve for y by adding 7 to both sides x + 7 = 3y, then divide by 3: y = (x + 7)/3. Choice C correctly finds f⁻¹(x) = (x + 7)/3 by swapping and solving properly. Choice D fails by confusing the inverse with a reciprocal form, likely from incorrectly solving the equation. The swap-and-solve recipe: (1) Replace f(x) with y to get y = 3x - 7, (2) Swap every x with y and every y with x: x = 3y - 7, (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 $f(x)=2x^3$.
$f^{-1}(x)=\dfrac{x}{2^3}$
$f^{-1}(x)=2\sqrt[3]{x}$
$f^{-1}(x)=\dfrac{1}{2x^3}$
$f^{-1}(x)=\sqrt[3]{\dfrac{x}{2}}$
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 f(x) = 2x³, we write y = 2x³, swap to get x = 2y³, then solve for y: divide both sides by 2 to get x/2 = y³, then take the cube root of both sides to get y = ∛(x/2), so f⁻¹(x) = ∛(x/2). Choice B correctly finds f⁻¹(x) = ∛(x/2) by swapping and solving properly—it undoes 'multiply by 2 then cube' with 'divide by 2 then take cube root.' Choice A incorrectly divides by 2³ = 8 instead of taking the cube root, while Choice D confuses inverse with reciprocal (1/(2x³) is NOT the inverse function!). The swap-and-solve recipe ensures you reverse the function's operations in the correct order: if f does 'multiply by 2, then cube,' the inverse must 'uncube (cube root), then divide by 2.'
Determine the inverse function for $f(x)=3x-7$.
$f^{-1}(x)=\dfrac{x+7}{3}$
$f^{-1}(x)=\dfrac{x-7}{3}$
$f^{-1}(x)=\dfrac{3}{x-7}$
$f^{-1}(x)=3x+7$
Explanation
This question tests your ability to find inverse functions—functions that undo what the original function does, reversing the input-output relationship. 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) = 3x - 7, let's apply swap-and-solve: Start with y = 3x - 7, swap to get x = 3y - 7, add 7 to both sides to get x + 7 = 3y, then divide by 3 to get y = (x + 7)/3. Choice C correctly finds f⁻¹(x) = (x + 7)/3 by swapping and solving properly. Choice A gives 3x + 7, which would be composing the function with itself rather than finding its inverse; Choice B incorrectly subtracts 7 instead of adding; Choice D mistakenly treats this as a reciprocal function. The swap-and-solve recipe: (1) Replace f(x) with y to get y = 3x - 7, (2) Swap every x with y and every y with x: x = 3y - 7, (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. Inverse thinking: ask yourself 'what operations does f do, and in what order?' then reverse the order and undo each operation. If f(x) = 3x - 7 does 'multiply by 3, then subtract 7,' the inverse should do 'add 7, then divide by 3': (x + 7)/3. This intuitive approach helps you predict what the inverse should be before computing it algebraically!
Find the inverse function $f^{-1}(x)$ for $f(x)=2x^3$.
$f^{-1}(x)=\frac{1}{2x^3}$
$f^{-1}(x)=2\sqrt[3]{x}$
$f^{-1}(x)=\sqrt[3]{\frac{x}{2}}$
$f^{-1}(x)=\sqrt[3]{2x}$
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) = 2x³, start by writing y = 2x³, swap x and y to get x = 2y³, solve for y by dividing both sides by 2 to get y³ = x/2, and then take the cube root: y = sqrt[3]{x/2}. Choice A correctly finds f⁻¹(x) = sqrt[3]{x/2} by swapping and solving properly. Choice D fails because it confuses the inverse with the reciprocal, giving 1/(2x³), which does not undo the original function. The swap-and-solve recipe: (1) Replace f(x) with y to get y = 2x³, (2) Swap every x with y and every y with x: x = 2y³, (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 $f(x)=\dfrac{x-4}{3}$.
$f^{-1}(x)=\dfrac{3}{x-4}$
$f^{-1}(x)=\dfrac{x-4}{3}$
$f^{-1}(x)=3x+4$
$f^{-1}(x)=3x-4$
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) = 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.