Derivative of \( \displaystyle \frac{\ln{\left(\cos{\left(3 x \right)} - 1 \right)}}{3} - \frac{\ln{\left(\cos{\left(3 x \right)} + 1 \right)}}{3} \)
Problem 2.701 · hard Beautiful
Differentiate \( \displaystyle f(x) = \frac{\ln{\left(\cos{\left(3 x \right)} - 1 \right)}}{3} - \frac{\ln{\left(\cos{\left(3 x \right)} + 1 \right)}}{3} \).
- \[ \frac{d}{d x} \left(\frac{\ln{\left(\cos{\left(3 x \right)} - 1 \right)}}{3} - \frac{\ln{\left(\cos{\left(3 x \right)} + 1 \right)}}{3}\right) \]derivativeStart with the derivative of the function.✓ Proved
- \[ = \frac{d}{d x} \frac{\ln{\left(\cos{\left(3 x \right)} - 1 \right)}}{3} - \frac{d}{d x} \frac{\ln{\left(\cos{\left(3 x \right)} + 1 \right)}}{3} \]sumApply the difference rule.✓ Proved
- \[ = \frac{\frac{d}{d x} \ln{\left(\cos{\left(3 x \right)} - 1 \right)}}{3} - \frac{\frac{d}{d x} \ln{\left(\cos{\left(3 x \right)} + 1 \right)}}{3} \]constant-multipleFactor out the constant 1/3.✓ Proved
- \[ = - \frac{\frac{d}{d x} \left(\cos{\left(3 x \right)} + 1\right)}{3 \left(\cos{\left(3 x \right)} + 1\right)} + \frac{\frac{d}{d x} \left(\cos{\left(3 x \right)} - 1\right)}{3 \left(\cos{\left(3 x \right)} - 1\right)} \]chainApply the chain rule to the logarithmic terms.✓ Proved
- \[ = - \frac{\frac{d}{d x} \cos{\left(3 x \right)}}{3 \left(\cos{\left(3 x \right)} + 1\right)} + \frac{\frac{d}{d x} \cos{\left(3 x \right)}}{3 \left(\cos{\left(3 x \right)} - 1\right)} \]derivativeDifferentiate the inner terms.✓ Proved
- \[ = \frac{\sin{\left(3 x \right)}}{\cos{\left(3 x \right)} + 1} - \frac{\sin{\left(3 x \right)}}{\cos{\left(3 x \right)} - 1} \]trig algebraDifferentiate the cosine function. Simplify the coefficients.✓ Proved
- \[ = \left(\frac{1}{\cos{\left(3 x \right)} + 1} - \frac{1}{\cos{\left(3 x \right)} - 1}\right) \sin{\left(3 x \right)} \]algebraFactor out sin(3*x).✓ Proved
- \[ = - \frac{2 \sin{\left(3 x \right)}}{\left(\cos{\left(3 x \right)} - 1\right) \left(\cos{\left(3 x \right)} + 1\right)} \]algebraFind a common denominator.✓ Proved
- \[ = - \frac{2 \sin{\left(3 x \right)}}{\cos^{2}{\left(3 x \right)} - 1} \]algebra algebraSimplify the numerator and denominator. Distribute sin(3*x).✓ Proved
- \[ = \frac{2}{\sin{\left(3 x \right)}} \]algebra simplifyUse the identity sin(x)**2 = 1 - cos(x)**2. Cancel sin(3*x) and simplify the sign.✓ Proved
- \[ = 2 \csc{\left(3 x \right)} \]rewriteRewrite using the cosecant function.✓ Proved
Answer \( \frac{2}{\sin{\left(3 x \right)}} \)
Mind the domain. The answer is also defined at points where f(x) is not. Substituting there gives a number that is not a slope of f.
✓ 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 undefined where cos(3*x) + 1 = 0 undefined where cos(3*x) - 1 = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(3*x) + 1 = 0 undefined where cos(3*x) - 1 = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(3*x) + 1 = 0 undefined where cos(3*x) - 1 = 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(3*x) + 1 = 0 undefined where cos(3*x) - 1 = 0 |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(3*x) + 1 = 0 undefined where cos(3*x) - 1 = 0 |
| 9 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(3*x) + 1 = 0 undefined where cos(3*x) - 1 = 0 |
| 10 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(3*x) + 1 = 0 undefined where cos(3*x) - 1 = 0 undefined where cos(3*x)**2 - 1 = 0 |
| 11 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(3*x)**2 - 1 = 0 |
| 12 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(3*x)**2 - 1 = 0 undefined where sin(3*x) = 0 |
| 13 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where sin(3*x) = 0 |
| 14 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where sin(3*x) = 0 csc has poles at multiples of pi |
| answer | ✓ Proved | sympy 1.14.0 | final line against the stated answer: simplify(a - b) reduced to 0 undefined where sin(3*x) = 0 |
| 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 and algebraic simplifications in single-step increments. The labels accurately reflect the operations performed, and the final result matches the stated answer.
Every verdict on record (4)
qwen3.6:27b-mlx: pass 2026-09-21 — The solution correctly applies differentiation rules and algebraic simplifications in single-step increments. The labels accurately reflect the operations performed, and the final result matches the stated answer.gpt-oss:20b: pass 2026-09-21qwen3.6:27b-mlx: pass 2026-09-21 — 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 2026-09-21
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.