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 \( \displaystyle f(x) = x \ln{\left(4 x + 2 \right)} - x + \frac{\ln{\left(2 x + 1 \right)}}{2} \).
- \[ \frac{d}{d x} \left(x \ln{\left(4 x + 2 \right)} - x + \frac{\ln{\left(2 x + 1 \right)}}{2}\right) \]Start with the derivative of the function.✓ Proved
- \[ = - \frac{d}{d x} x + \frac{d}{d x} x \ln{\left(4 x + 2 \right)} + \frac{d}{d x} \frac{\ln{\left(2 x + 1 \right)}}{2} \]sumApply the sum rule to separate the terms.✓ Proved
- \[ = \frac{d}{d x} x \ln{\left(4 x + 2 \right)} + \frac{d}{d x} \frac{\ln{\left(2 x + 1 \right)}}{2} - 1 \]derivativeDifferentiate the term x.✓ Proved
- \[ = x \frac{d}{d x} \ln{\left(4 x + 2 \right)} + \ln{\left(4 x + 2 \right)} + \frac{d}{d x} \frac{\ln{\left(2 x + 1 \right)}}{2} - 1 \]productApply the product rule to the first term.✓ Proved
- \[ = \frac{4 x}{4 x + 2} + \ln{\left(4 x + 2 \right)} + \frac{d}{d x} \frac{\ln{\left(2 x + 1 \right)}}{2} - 1 \]chain algebraApply the chain rule to log(4*x + 2). Multiply the numerator.✓ Proved
- \[ = \frac{4 x}{4 x + 2} + \ln{\left(4 x + 2 \right)} + \frac{\frac{d}{d x} \ln{\left(2 x + 1 \right)}}{2} - 1 \]constant-multiplePull out the constant 1/2.✓ Proved
- \[ = \frac{4 x}{4 x + 2} + \ln{\left(4 x + 2 \right)} - 1 + \frac{1}{2 x + 1} \]chain algebraApply the chain rule to log(2*x + 1). Simplify the constant multiplication.✓ Proved
- \[ = \frac{2 x}{2 x + 1} + \ln{\left(4 x + 2 \right)} - 1 + \frac{1}{2 x + 1} \]algebraSimplify the fraction (4*x)/(4*x + 2) by dividing by 2.✓ Proved
- \[ = \ln{\left(4 x + 2 \right)} \]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
| 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 undefined where 4*x + 2 = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments undefined where 4*x + 2 = 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments undefined where 4*x + 2 = 0 |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(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 | ✓ Proved | sympy 1.14.0 | simplify(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 | ✓ Proved | sympy 1.14.0 | simplify(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 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments undefined where 2*x + 1 = 0 |
| 12 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments |
| 13 | ✓ 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
deepseek-r1:70b: passgpt-oss:20b: passqwen3.6:27b-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 2026-09-20qwen3.6:27b-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 2026-09-20qwen3.6:27b-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 2026-09-19gpt-oss:20b: pass 2026-09-19qwen3.6:27b-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 2026-09-19gpt-oss:20b: pass 2026-09-19deepseek-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.