∫Calc Practice

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

Problem 2.963 · hard Beautiful

Differentiate \( \displaystyle f(x) = x \ln{\left(x - 1 \right)} - x - \ln{\left(x - 1 \right)} \).
  1. \[ \frac{d}{d x} \left(x \ln{\left(x - 1 \right)} - x - \ln{\left(x - 1 \right)}\right) \]
    Start with the derivative of the function.✓ Proved
  2. \[ = - \frac{d}{d x} x + \frac{d}{d x} x \ln{\left(x - 1 \right)} - \frac{d}{d x} \ln{\left(x - 1 \right)} \]
    sumApply the sum rule for differentiation.✓ Proved
  3. \[ = \frac{d}{d x} x \ln{\left(x - 1 \right)} - \frac{d}{d x} \ln{\left(x - 1 \right)} - 1 \]
    constantThe derivative of x is 1.✓ Proved
  4. \[ = x \frac{d}{d x} \ln{\left(x - 1 \right)} + \ln{\left(x - 1 \right)} \frac{d}{d x} x - \frac{d}{d x} \ln{\left(x - 1 \right)} - 1 \]
    productApply the product rule to the first term.✓ Proved
  5. \[ = x \frac{d}{d x} \ln{\left(x - 1 \right)} + \ln{\left(x - 1 \right)} - \frac{d}{d x} \ln{\left(x - 1 \right)} - 1 \]
    constantThe derivative of x is 1.✓ Proved
  6. \[ = \frac{x}{x - 1} + \ln{\left(x - 1 \right)} - 1 - \frac{1}{x - 1} \]
    logarithmic algebraApply the derivative rule for the logarithm. Simplify the expression.✓ Proved
  7. \[ = \ln{\left(x - 1 \right)} \]
    algebra algebra simplifyCombine the fractions with the same denominator. Simplify the fraction. Final simplification.✓ Proved
Answer \( \ln{\left(x - 1 \right)} \)

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
undefined where x - 1 = 0
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
10✓ 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: inconclusive — reviewer response could not be parsed: {"verdict":"fail","severity":"style","notes":"The solution incorrectly labels the derivative of \(x\) as a \"constant\" rule in steps 3 and 5. The correct label for that operation is \"derivative\". N
  • qwen3.6:27b-mlx: pass
Every verdict on record (4)
  • qwen3.6:27b-mlx: pass 2026-09-27
  • gpt-oss:20b: inconclusive 2026-09-27 — reviewer response could not be parsed: {"verdict":"fail","severity":"style","notes":"The solution incorrectly labels the derivative of \(x\) as a \"constant\" rule in steps 3 and 5. The correct label for that operation is \"derivative\". N
  • qwen3.6:27b-mlx: pass 2026-09-27
  • gpt-oss:20b: pass 2026-09-27

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 2026-09-27 with SymPy 1.14.0.