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Derivative of \( \displaystyle x \ln{\left(4 x + 2 \right)} - x + \frac{\ln{\left(2 x + 1 \right)}}{2} \)

Problem 2.6 · hard Beautiful

Differentiate \( \displaystyle f(x) = x \ln{\left(4 x + 2 \right)} - x + \frac{\ln{\left(2 x + 1 \right)}}{2} \).
  1. \[ \frac{d}{d x} \left(x \ln{\left(4 x + 2 \right)} - x + \frac{\ln{\left(2 x + 1 \right)}}{2}\right) \]
    Start with the derivative of the function.✓ Proved
  2. \[ = - \frac{d}{d x} x + \frac{d}{d x} x \ln{\left(4 x + 2 \right)} + \frac{d}{d x} \frac{\ln{\left(2 x + 1 \right)}}{2} \]
    sumApply the sum rule to separate the terms.✓ Proved
  3. \[ = \frac{d}{d x} x \ln{\left(4 x + 2 \right)} + \frac{d}{d x} \frac{\ln{\left(2 x + 1 \right)}}{2} - 1 \]
    derivativeDifferentiate the term x.✓ Proved
  4. \[ = x \frac{d}{d x} \ln{\left(4 x + 2 \right)} + \ln{\left(4 x + 2 \right)} + \frac{d}{d x} \frac{\ln{\left(2 x + 1 \right)}}{2} - 1 \]
    productApply the product rule to the first term.✓ Proved
  5. \[ = \frac{4 x}{4 x + 2} + \ln{\left(4 x + 2 \right)} + \frac{d}{d x} \frac{\ln{\left(2 x + 1 \right)}}{2} - 1 \]
    chain algebraApply the chain rule to log(4*x + 2). Multiply the numerator.✓ Proved
  6. \[ = \frac{4 x}{4 x + 2} + \ln{\left(4 x + 2 \right)} + \frac{\frac{d}{d x} \ln{\left(2 x + 1 \right)}}{2} - 1 \]
    constant-multiplePull out the constant 1/2.✓ Proved
  7. \[ = \frac{4 x}{4 x + 2} + \ln{\left(4 x + 2 \right)} - 1 + \frac{1}{2 x + 1} \]
    chain algebraApply the chain rule to log(2*x + 1). Simplify the constant multiplication.✓ Proved
  8. \[ = \frac{2 x}{2 x + 1} + \ln{\left(4 x + 2 \right)} - 1 + \frac{1}{2 x + 1} \]
    algebraSimplify the fraction (4*x)/(4*x + 2) by dividing by 2.✓ Proved
  9. \[ = \ln{\left(4 x + 2 \right)} \]
    algebra simplify simplifyCombine the fractions with the same denominator. Simplify the fraction (2*x + 1)/(2*x + 1). Combine the constant terms.✓ Proved
Answer \( \log{\left(4 x + 2 \right)} \)

✓ 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
undefined where 4*x + 2 = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where 4*x + 2 = 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where 4*x + 2 = 0
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where 4*x + 2 = 0
undefined where 2*x + 1 = 0
9✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where 4*x + 2 = 0
undefined where 2*x + 1 = 0
10✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where 4*x + 2 = 0
undefined where 2*x + 1 = 0
11✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where 2*x + 1 = 0
12✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
13✓ 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

  • deepseek-r1:70b: pass
  • gpt-oss:20b: pass
  • qwen3.6:27b-mlx: pass — The solution correctly applies differentiation rules one at a time and uses valid labels from the fixed vocabulary. The algebraic simplifications are sound.
Every verdict on record (11)
  • qwen3.6:27b-mlx: pass 2026-09-20 — The solution correctly applies differentiation rules one at a time and uses valid labels from the fixed vocabulary. The algebraic simplifications are sound.
  • gpt-oss:20b: pass 2026-09-20
  • qwen3.6:27b-mlx: pass 2026-09-20 — The solution correctly applies differentiation rules and algebraic simplifications. Each step changes only one aspect of the expression, and the labels accurately reflect the operations performed.
  • gpt-oss:20b: pass 2026-09-20
  • qwen3.6:27b-mlx: pass 2026-09-19 — The solution correctly applies differentiation rules and algebraic simplifications in single-step increments. All labels are valid and accurately describe 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 — The solution correctly applies differentiation rules step-by-step, adhering to the one-change-per-step constraint. The algebraic simplifications are valid and clearly labeled.
  • deepseek-r1:70b: pass 2026-09-19
  • gpt-oss:20b: pass 2026-09-19
  • deepseek-r1:70b: pass 2026-09-16

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.