Inverse Functions
Finding and understanding inverse functions
Inverse Functions
Definition
is the inverse of if:
The inverse "undoes" what the function does.
Finding an Inverse
Steps:
- Write
- Swap and
- Solve for
- Replace with
Example: Find inverse of
- , so
Domain and Range
- Domain of = Range of
- Range of = Domain of
Graphing
The graph of is the reflection of over the line .
Horizontal Line Test
has an inverse function if and only if no horizontal line intersects the graph more than once.
Verifying
To verify is the inverse of :
Check that AND
๐ Practice Problems
1Problem 1easy
โ Question:
Find the inverse: f(x) = 2x + 3
๐ก 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 fโปยน(x): fโปยน(x) = (x - 3)/2
Step 5: Verify (check f(fโปยน(x)) = x): f(fโปยน(x)) = 2((x - 3)/2) + 3 = (x - 3) + 3 = x โ
Answer: fโปยน(x) = (x - 3)/2
2Problem 2easy
โ Question:
Find the inverse of
๐ก Show Solution
Step 1: Write as
Step 2: Swap and
Step 3: Solve for
Step 4: Write inverse
Answer:
3Problem 3easy
โ Question:
Verify that f(x) = xยณ and g(x) = โx are inverse functions.
๐ก Show Solution
Step 1: Check f(g(x)) = x: f(g(x)) = f(โx) = (โx)ยณ = x โ
Step 2: Check g(f(x)) = x: g(f(x)) = g(xยณ) = โ(xยณ) = x โ
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)
๐ก 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: fโปยน(x) = (2x + 1)/(x - 1)
Answer: fโปยน(x) = (2x + 1)/(x - 1)
5Problem 5medium
โ Question:
Find the inverse of
๐ก Show Solution
Step 1: Write as
Step 2: Swap and
Step 3: Solve for
Step 4: Write inverse
Verify: โ
Answer:
6Problem 6hard
โ Question:
Find the inverse of
๐ก Show Solution
Step 1: Write as
Step 2: Swap and
Step 3: Solve for (multiply both sides by denominator)
Group terms:
Answer:
7Problem 7medium
โ Question:
Find the inverse and state the domain and range: f(x) = โ(x - 3)
๐ก Show Solution
Step 1: Replace f(x) with y: y = โ(x - 3)
Step 2: Identify domain and range of f: Domain of f: x โฅ 3 Range of f: y โฅ 0
Step 3: Swap x and y: x = โ(y - 3)
Step 4: Solve for y: xยฒ = y - 3 y = xยฒ + 3
Step 5: Write inverse: fโปยน(x) = xยฒ + 3
Step 6: Domain and range of fโปยน: Domain of fโปยน = Range of f: x โฅ 0 Range of fโปยน = Domain of f: y โฅ 3
Answer: fโปยน(x) = xยฒ + 3, Domain: x โฅ 0, Range: y โฅ 3
8Problem 8hard
โ Question:
Determine if f(x) = xยฒ has an inverse function. If not, restrict the domain so it does.
๐ก 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 โฅ 0 (right half) OR restrict to x โค 0 (left half)
Step 4: Find inverse with restriction x โฅ 0: y = xยฒ, x โฅ 0 x = yยฒ, y โฅ 0 y = โx
Step 5: Verify: fโปยน(x) = โx, Domain: x โฅ 0
Answer: No inverse without restriction. With x โฅ 0: fโปยน(x) = โx
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