Derivative of \( \displaystyle 2 x \ln{\left(x + 1 \right)} - 2 x + 2 \ln{\left(x + 1 \right)} \)
Problem 2.1129 · hard Beautiful
Differentiate \( \displaystyle f(x) = 2 x \ln{\left(x + 1 \right)} - 2 x + 2 \ln{\left(x + 1 \right)} \).
- \[ \frac{d}{d x} \left(2 x \ln{\left(x + 1 \right)} - 2 x + 2 \ln{\left(x + 1 \right)}\right) \]Differentiate the function with respect to x.✓ Proved
- \[ = - \frac{d}{d x} 2 x + \frac{d}{d x} 2 x \ln{\left(x + 1 \right)} + \frac{d}{d x} 2 \ln{\left(x + 1 \right)} \]sumApply the sum rule for differentiation.✓ Proved
- \[ = \frac{d}{d x} 2 x \ln{\left(x + 1 \right)} + \frac{d}{d x} 2 \ln{\left(x + 1 \right)} - 2 \]constantThe derivative of 2*x is 2.✓ Proved
- \[ = 2 \frac{d}{d x} x \ln{\left(x + 1 \right)} + 2 \frac{d}{d x} \ln{\left(x + 1 \right)} - 2 \]constant-multipleFactor out the constant 2 from the terms.✓ Proved
- \[ = 2 x \frac{d}{d x} \ln{\left(x + 1 \right)} + 2 \ln{\left(x + 1 \right)} \frac{d}{d x} x + 2 \frac{d}{d x} \ln{\left(x + 1 \right)} - 2 \]productApply the product rule to the first term.✓ Proved
- \[ = 2 x \frac{d}{d x} \ln{\left(x + 1 \right)} + 2 \ln{\left(x + 1 \right)} + 2 \frac{d}{d x} \ln{\left(x + 1 \right)} - 2 \]derivativeThe derivative of x is 1.✓ Proved
- \[ = \frac{2 x}{x + 1} + 2 \ln{\left(x + 1 \right)} - 2 + \frac{2}{x + 1} \]derivative algebraThe derivative of log(x + 1) is 1/(x + 1). Distribute the 2.✓ Proved
- \[ = 2 \ln{\left(x + 1 \right)} - 2 + \frac{2 x + 2}{x + 1} \]algebra algebraCombine the fractions with the same denominator. Factor the numerator of the fraction.✓ Proved
- \[ = 2 \ln{\left(x + 1 \right)} \]algebra simplifySimplify the fraction by canceling (x + 1). Simplify the remaining terms.✓ Proved
Answer \( 2 \ln{\left(x + 1 \right)} \)
Every line of this solution was proved by the computer algebra system SymPy. 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.
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | ✓ Proved | sympy 1.14.0 | line 1 is the problem as stated |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments undefined where x + 1 = 0 |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments undefined where x + 1 = 0 |
| 9 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments undefined where x + 1 = 0 |
| 10 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments undefined where x + 1 = 0 |
| 11 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments undefined where x + 1 = 0 |
| 12 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments |
| answer | ✓ Proved | sympy 1.14.0 | final line against the stated answer: simplify(a - b) reduced to 0 log is undefined for non-positive arguments |
| answer, a second way | ✓ Proved | sympy 1.14.0 | SymPy differentiated f directly and got the stated answer |
Reviewers
gpt-oss:20b: fail (error) — Step 3 applies both the constant‑multiple rule (2*x → 2*1) and the derivative rule (d/dx x = 1), but labels it only as "constant". This violates the one‑rule‑per‑step requirement.qwen3.6:27b-mlx: pass
Every verdict on record (4)
qwen3.6:27b-mlx: pass 2026-09-28gpt-oss:20b: fail (error) 2026-09-28 — Step 3 applies both the constant‑multiple rule (2*x → 2*1) and the derivative rule (d/dx x = 1), but labels it only as "constant". This violates the one‑rule‑per‑step requirement.qwen3.6:27b-mlx: pass 2026-09-28gpt-oss:20b: pass 2026-09-28
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
gemma4:26b, checked 2026-09-28 with SymPy 1.14.0.