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Derivative of \( \displaystyle x \left(1 - \ln{\left(3 x \right)}\right) \)

Problem 2.164 · hard

Differentiate \( \displaystyle f(x) = x \left(1 - \ln{\left(3 x \right)}\right) \).
  1. \[ \frac{d}{d x} x \left(1 - \ln{\left(3 x \right)}\right) \]
    derivativeStart with the derivative of the function.✓ Proved
  2. \[ = x \frac{d}{d x} \left(1 - \ln{\left(3 x \right)}\right) + \left(1 - \ln{\left(3 x \right)}\right) \frac{d}{d x} x \]
    productApply the product rule.✓ Proved
  3. \[ = x \frac{d}{d x} \left(1 - \ln{\left(3 x \right)}\right) - \ln{\left(3 x \right)} + 1 \]
    constantThe derivative of x is 1.✓ Proved
  4. \[ = x \left(\frac{d}{d x} 1 - \frac{d}{d x} \ln{\left(3 x \right)}\right) - \ln{\left(3 x \right)} + 1 \]
    sumApply the derivative to the sum inside the parentheses.✓ Proved
  5. \[ = - x \frac{d}{d x} \ln{\left(3 x \right)} - \ln{\left(3 x \right)} + 1 \]
    constant algebraThe derivative of 1 is 0. Simplify the expression inside the parentheses.✓ Proved
  6. \[ = - \ln{\left(3 x \right)} - \frac{\frac{d}{d x} 3 x}{3} + 1 \]
    chainApply the chain rule to the logarithm.✓ Proved
  7. \[ = - \ln{\left(3 x \right)} \]
    derivative algebra algebra simplifyThe derivative of 3*x is 3. Simplify the fraction. Multiply x by -1/x. Combine the constant terms.✓ Proved
Answer \( - \log{\left(3 x \right)} \)

✓ Nihil obstat 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. Reviewers found nothing wrong with the explanation.

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
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
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
11✓ 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: pass
  • deepseek-r1:70b: pass
  • qwen3.6:27b-mlx: pass
Every verdict on record (15)
  • qwen3.6:27b-mlx: pass 2026-09-20
  • gpt-oss:20b: pass 2026-09-20
  • qwen3.6:27b-mlx: pass 2026-09-20 — The solution correctly applies the product rule, chain rule, and algebraic simplifications in distinct steps. All labels correspond to the rules applied.
  • gpt-oss:20b: pass 2026-09-20
  • qwen3.6:27b-mlx: pass 2026-09-19 — The solution correctly applies the product rule, chain rule, and algebraic simplifications in distinct steps. All labels are appropriate for the operations performed.
  • deepseek-r1:70b: pass 2026-09-19
  • gpt-oss:20b: pass 2026-09-19
  • qwen3.6:27b-mlx: pass 2026-09-19
  • deepseek-r1:70b: pass 2026-09-19
  • gpt-oss:20b: pass 2026-09-19
  • qwen3.6:27b-mlx: pass 2026-09-18 — The solution correctly applies the product rule, chain rule, and algebraic simplifications in distinct steps. All labels are appropriate for the transformations performed.
  • deepseek-r1:70b: pass 2026-09-18
  • gpt-oss:20b: pass 2026-09-18
  • gpt-oss:20b: fail 2026-09-17 — Step 3 incorrectly labels the rule as 'constant' when it is actually the derivative of x (the identity function).
  • deepseek-r1:70b: fail 2026-09-17 — Step 9 incorrectly simplifies x*(-1/(3*x)) to -1 instead of -1/3.

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 2026-09-26 with SymPy 1.14.0.