Inverse Functions

Finding and understanding inverse functions

Inverse Functions

Definition

fโˆ’1f^{-1} is the inverse of ff if: f(fโˆ’1(x))=xย andย fโˆ’1(f(x))=xf(f^{-1}(x)) = x \text{ and } f^{-1}(f(x)) = x

The inverse "undoes" what the function does.

Finding an Inverse

Steps:

  1. Write y=f(x)y = f(x)
  2. Swap xx and yy
  3. Solve for yy
  4. Replace yy with fโˆ’1(x)f^{-1}(x)

Example: Find inverse of f(x)=2x+3f(x) = 2x + 3

  1. y=2x+3y = 2x + 3
  2. x=2y+3x = 2y + 3
  3. xโˆ’3=2yx - 3 = 2y, so y=xโˆ’32y = \frac{x - 3}{2}
  4. fโˆ’1(x)=xโˆ’32f^{-1}(x) = \frac{x - 3}{2}

Domain and Range

  • Domain of ff = Range of fโˆ’1f^{-1}
  • Range of ff = Domain of fโˆ’1f^{-1}

Graphing

The graph of fโˆ’1f^{-1} is the reflection of ff over the line y=xy = x.

Horizontal Line Test

ff has an inverse function if and only if no horizontal line intersects the graph more than once.

Verifying

To verify gg is the inverse of ff:

Check that f(g(x))=xf(g(x)) = x AND g(f(x))=xg(f(x)) = x

๐Ÿ“š Practice Problems

1Problem 1easy

โ“ Question:

Find the inverse: f(x) = 2x + 3

๐Ÿ’ก Show Solution

Step 1: Replace f(x) with y: y = 2x + 3

Step 2: Swap x and y: x = 2y + 3

Step 3: Solve for y: x - 3 = 2y y = (x - 3)/2

Step 4: Replace y with fโปยน(x): fโปยน(x) = (x - 3)/2

Step 5: Verify (check f(fโปยน(x)) = x): f(fโปยน(x)) = 2((x - 3)/2) + 3 = (x - 3) + 3 = x โœ“

Answer: fโปยน(x) = (x - 3)/2

2Problem 2easy

โ“ Question:

Find the inverse of f(x)=x+7f(x) = x + 7

๐Ÿ’ก Show Solution

Step 1: Write as y=x+7y = x + 7

Step 2: Swap xx and yy x=y+7x = y + 7

Step 3: Solve for yy y=xโˆ’7y = x - 7

Step 4: Write inverse fโˆ’1(x)=xโˆ’7f^{-1}(x) = x - 7

Answer: fโˆ’1(x)=xโˆ’7f^{-1}(x) = x - 7

3Problem 3easy

โ“ Question:

Verify that f(x) = xยณ and g(x) = โˆ›x are inverse functions.

๐Ÿ’ก Show Solution

Step 1: Check f(g(x)) = x: f(g(x)) = f(โˆ›x) = (โˆ›x)ยณ = x โœ“

Step 2: Check g(f(x)) = x: g(f(x)) = g(xยณ) = โˆ›(xยณ) = x โœ“

Step 3: Conclusion: Since both compositions equal x, f and g are inverse functions

Answer: Yes, they are inverse functions

4Problem 4medium

โ“ Question:

Find the inverse: f(x) = (x + 1)/(x - 2)

๐Ÿ’ก Show Solution

Step 1: Replace f(x) with y: y = (x + 1)/(x - 2)

Step 2: Swap x and y: x = (y + 1)/(y - 2)

Step 3: Solve for y (multiply both sides by (y - 2)): x(y - 2) = y + 1 xy - 2x = y + 1

Step 4: Collect y terms: xy - y = 2x + 1 y(x - 1) = 2x + 1

Step 5: Solve for y: y = (2x + 1)/(x - 1)

Step 6: Write inverse: fโปยน(x) = (2x + 1)/(x - 1)

Answer: fโปยน(x) = (2x + 1)/(x - 1)

5Problem 5medium

โ“ Question:

Find the inverse of f(x)=xโˆ’13f(x) = \frac{x - 1}{3}

๐Ÿ’ก Show Solution

Step 1: Write as y=xโˆ’13y = \frac{x - 1}{3}

Step 2: Swap xx and yy x=yโˆ’13x = \frac{y - 1}{3}

Step 3: Solve for yy 3x=yโˆ’13x = y - 1 y=3x+1y = 3x + 1

Step 4: Write inverse fโˆ’1(x)=3x+1f^{-1}(x) = 3x + 1

Verify: f(fโˆ’1(x))=(3x+1)โˆ’13=3x3=xf(f^{-1}(x)) = \frac{(3x + 1) - 1}{3} = \frac{3x}{3} = x โœ“

Answer: fโˆ’1(x)=3x+1f^{-1}(x) = 3x + 1

6Problem 6hard

โ“ Question:

Find the inverse of f(x)=2x+3xโˆ’1f(x) = \frac{2x + 3}{x - 1}

๐Ÿ’ก Show Solution

Step 1: Write as y=2x+3xโˆ’1y = \frac{2x + 3}{x - 1}

Step 2: Swap xx and yy x=2y+3yโˆ’1x = \frac{2y + 3}{y - 1}

Step 3: Solve for yy (multiply both sides by denominator) x(yโˆ’1)=2y+3x(y - 1) = 2y + 3 xyโˆ’x=2y+3xy - x = 2y + 3

Group yy terms: xyโˆ’2y=x+3xy - 2y = x + 3 y(xโˆ’2)=x+3y(x - 2) = x + 3 y=x+3xโˆ’2y = \frac{x + 3}{x - 2}

Answer: fโˆ’1(x)=x+3xโˆ’2f^{-1}(x) = \frac{x + 3}{x - 2}

7Problem 7medium

โ“ Question:

Find the inverse and state the domain and range: f(x) = โˆš(x - 3)

๐Ÿ’ก Show Solution

Step 1: Replace f(x) with y: y = โˆš(x - 3)

Step 2: Identify domain and range of f: Domain of f: x โ‰ฅ 3 Range of f: y โ‰ฅ 0

Step 3: Swap x and y: x = โˆš(y - 3)

Step 4: Solve for y: xยฒ = y - 3 y = xยฒ + 3

Step 5: Write inverse: fโปยน(x) = xยฒ + 3

Step 6: Domain and range of fโปยน: Domain of fโปยน = Range of f: x โ‰ฅ 0 Range of fโปยน = Domain of f: y โ‰ฅ 3

Answer: fโปยน(x) = xยฒ + 3, Domain: x โ‰ฅ 0, Range: y โ‰ฅ 3

8Problem 8hard

โ“ Question:

Determine if f(x) = xยฒ has an inverse function. If not, restrict the domain so it does.

๐Ÿ’ก Show Solution

Step 1: Apply horizontal line test: Does any horizontal line intersect y = xยฒ more than once? Yes - for example, y = 4 intersects at x = 2 and x = -2

Step 2: Conclusion about inverse: f(x) = xยฒ does NOT have an inverse (not one-to-one)

Step 3: Restrict domain to make it one-to-one: Restrict to x โ‰ฅ 0 (right half) OR restrict to x โ‰ค 0 (left half)

Step 4: Find inverse with restriction x โ‰ฅ 0: y = xยฒ, x โ‰ฅ 0 x = yยฒ, y โ‰ฅ 0 y = โˆšx

Step 5: Verify: fโปยน(x) = โˆšx, Domain: x โ‰ฅ 0

Answer: No inverse without restriction. With x โ‰ฅ 0: fโปยน(x) = โˆšx