Derivative of \( \displaystyle - x \ln{\left(2 x + 1 \right)} + x - \frac{\ln{\left(2 x + 1 \right)}}{2} \)
Problem 2.1206 · hard
Differentiate \( \displaystyle f(x) = - x \ln{\left(2 x + 1 \right)} + x - \frac{\ln{\left(2 x + 1 \right)}}{2} \).
- \[ \frac{d}{d x} \left(- x \ln{\left(2 x + 1 \right)} + x - \frac{\ln{\left(2 x + 1 \right)}}{2}\right) \]derivativeStart with the derivative of the function.✓ Proved
- \[ = \frac{d}{d x} x + \frac{d}{d x} \left(- x \ln{\left(2 x + 1 \right)}\right) - \frac{d}{d x} \frac{\ln{\left(2 x + 1 \right)}}{2} \]sumApply the sum rule.✓ Proved
- \[ = \frac{d}{d x} \left(- x \ln{\left(2 x + 1 \right)}\right) - \frac{d}{d x} \frac{\ln{\left(2 x + 1 \right)}}{2} + 1 \]derivativeDifferentiate the term x.✓ Proved
- \[ = \frac{d}{d x} \left(- x \ln{\left(2 x + 1 \right)}\right) - \frac{\frac{d}{d x} \ln{\left(2 x + 1 \right)}}{2} + 1 \]constant-multiplePull out the constant factor 1/2.✓ Proved
- \[ = - x \frac{d}{d x} \ln{\left(2 x + 1 \right)} + \ln{\left(2 x + 1 \right)} \frac{d}{d x} \left(- x\right) - \frac{\frac{d}{d x} \ln{\left(2 x + 1 \right)}}{2} + 1 \]productApply the product rule to the first term.✓ Proved
- \[ = - x \frac{d}{d x} \ln{\left(2 x + 1 \right)} - \ln{\left(2 x + 1 \right)} - \frac{\frac{d}{d x} \ln{\left(2 x + 1 \right)}}{2} + 1 \]derivative algebraDifferentiate -x. Simplify the coefficient.✓ Proved
- \[ = - \left(x + \frac{1}{2}\right) \frac{d}{d x} \ln{\left(2 x + 1 \right)} - \ln{\left(2 x + 1 \right)} + 1 \]algebraGroup the terms containing the derivative.✓ Proved
- \[ = - \frac{\left(x + \frac{1}{2}\right) \frac{d}{d x} \left(2 x + 1\right)}{2 x + 1} - \ln{\left(2 x + 1 \right)} + 1 \]chainApply the chain rule to the logarithm.✓ Proved
- \[ = - \frac{2 \left(x + \frac{1}{2}\right)}{2 x + 1} - \ln{\left(2 x + 1 \right)} + 1 \]derivativeDifferentiate the inner function 2*x + 1.✓ Proved
- \[ = - \ln{\left(2 x + 1 \right)} \]algebra algebra algebraSimplify the product (x + 1/2) * (2 / (2x + 1)). Simplify the fraction. Final simplification.✓ Proved
Answer \( - \ln{\left(2 x + 1 \right)} \)
✓ 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 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments |
| 9 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 log is undefined for non-positive arguments 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 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
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly applies differentiation rules one at a time with appropriate labels. The algebraic simplifications are sound and clearly explained.
Every verdict on record (4)
qwen3.6:27b-mlx: pass 2026-09-29 — The solution correctly applies differentiation rules one at a time with appropriate labels. The algebraic simplifications are sound and clearly explained.gpt-oss:20b: pass 2026-09-29qwen3.6:27b-mlx: pass 2026-09-29gpt-oss:20b: pass 2026-09-29
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 2026-09-29 with SymPy 1.14.0.