Inverse functions
Problem 13.378 · easy
Find the inverse of \( \displaystyle f(x) = 3 - x \).
- Write y = f(x) and solve for x. Subtract the constant and divide. Then rename y as x.Reviewed
- \[ \left. 3 - x \right|_{\substack{ x=3 - x }} = x \]f(f⁻¹(x)) = x, so this is the inverse.✓ Proved
- \[ \left. 3 - x \right|_{\substack{ x=\left. 3 - x \right|_{\substack{ x=1 }} }} = 1 \]And f⁻¹(f(1)) = 1.✓ Proved
Answer \( f^{-1}(x) = 3 - x \)
Lines: 2 proved, 1 reviewed. The answer was also checked a second way, without looking at the solution. The reviewers disagree about how one step is explained; every verdict is in the receipt. How we verify
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | Reviewed | reviewers: gpt-oss:20b, qwen3.6:27b-mlx | a sentence; read, not computed |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| answer, a second way | ✓ Checked independently | sympy 1.14.0 + mpmath 1.3.0 | f then the answer, in floating point, returns the starting x |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly identifies that f(x) = 3 - x is its own inverse. The steps described in the sentence are valid, and the equations verify the result.
Every verdict on record (6)
qwen3.6:27b-mlx: pass 2026-10-19 — The solution correctly identifies that f(x) = 3 - x is its own inverse. The steps described in the sentence are valid, and the equations verify the result.gpt-oss:20b: pass 2026-10-19qwen3.6:27b-mlx: pass 2026-10-17 — The solution is correct and the reasoning is sound. The function f(x) = 3 - x is its own inverse, and the verification steps confirm this.gpt-oss:20b: pass 2026-10-17qwen3.6:27b-mlx: fail (error) 2026-10-17 — The solution claims f(x) = 3 - x is its own inverse, which is true, but the method described in step 1 ('Subtract the constant and divide') is incorrect for this function. Solving y = 3 - x for x yields x = 3 - y, not a division step. Furthermore, the 'equations' provided are not algebraic derivations but rather verification checks that do not constitute a valid method for finding the inverse.gpt-oss:20b: pass 2026-10-17
Proved: SymPy reduced the difference between the two sides to zero. Checked independently: a separate
method, named above, confirmed it. Checked numerically: the two sides agree at every sampled point, which is evidence,
not proof. Reviewed: a model or a person read it; that is all a sentence can have. Solution by
generator:structured/inverse_function, checked 2026-10-19 with SymPy 1.14.0.