∫Calc Practice

Derivative of \( \displaystyle 2 x \ln{\left(x + 1 \right)} - 2 x + 2 \ln{\left(x + 1 \right)} \)

Problem 2.1129 · hard Beautiful

Differentiate \( \displaystyle f(x) = 2 x \ln{\left(x + 1 \right)} - 2 x + 2 \ln{\left(x + 1 \right)} \).
  1. \[ \frac{d}{d x} \left(2 x \ln{\left(x + 1 \right)} - 2 x + 2 \ln{\left(x + 1 \right)}\right) \]
    Differentiate the function with respect to x.✓ Proved
  2. \[ = - \frac{d}{d x} 2 x + \frac{d}{d x} 2 x \ln{\left(x + 1 \right)} + \frac{d}{d x} 2 \ln{\left(x + 1 \right)} \]
    sumApply the sum rule for differentiation.✓ Proved
  3. \[ = \frac{d}{d x} 2 x \ln{\left(x + 1 \right)} + \frac{d}{d x} 2 \ln{\left(x + 1 \right)} - 2 \]
    constantThe derivative of 2*x is 2.✓ Proved
  4. \[ = 2 \frac{d}{d x} x \ln{\left(x + 1 \right)} + 2 \frac{d}{d x} \ln{\left(x + 1 \right)} - 2 \]
    constant-multipleFactor out the constant 2 from the terms.✓ Proved
  5. \[ = 2 x \frac{d}{d x} \ln{\left(x + 1 \right)} + 2 \ln{\left(x + 1 \right)} \frac{d}{d x} x + 2 \frac{d}{d x} \ln{\left(x + 1 \right)} - 2 \]
    productApply the product rule to the first term.✓ Proved
  6. \[ = 2 x \frac{d}{d x} \ln{\left(x + 1 \right)} + 2 \ln{\left(x + 1 \right)} + 2 \frac{d}{d x} \ln{\left(x + 1 \right)} - 2 \]
    derivativeThe derivative of x is 1.✓ Proved
  7. \[ = \frac{2 x}{x + 1} + 2 \ln{\left(x + 1 \right)} - 2 + \frac{2}{x + 1} \]
    derivative algebraThe derivative of log(x + 1) is 1/(x + 1). Distribute the 2.✓ Proved
  8. \[ = 2 \ln{\left(x + 1 \right)} - 2 + \frac{2 x + 2}{x + 1} \]
    algebra algebraCombine the fractions with the same denominator. Factor the numerator of the fraction.✓ Proved
  9. \[ = 2 \ln{\left(x + 1 \right)} \]
    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
LineStatusChecked byDetail
1✓ Provedsympy 1.14.0line 1 is the problem as stated
2✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 1 = 0
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 1 = 0
9✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 1 = 0
10✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 1 = 0
11✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 1 = 0
12✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
answer✓ Provedsympy 1.14.0final line against the stated answer: simplify(a - b) reduced to 0
log is undefined for non-positive arguments
answer, a second way✓ Provedsympy 1.14.0SymPy 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 2026-09-28
  • gpt-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 2026-09-28
  • gpt-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.