Derivative of \( \displaystyle \frac{5 \ln{\left(\cos^{2}{\left(x \right)} - 1 \right)}}{4} - \frac{5 \ln{\left(\cos^{2}{\left(x \right)} \right)}}{4} \)
Problem 2.1411 · hard Beautiful
Differentiate \( \displaystyle f(x) = \frac{5 \ln{\left(\cos^{2}{\left(x \right)} - 1 \right)}}{4} - \frac{5 \ln{\left(\cos^{2}{\left(x \right)} \right)}}{4} \).
- \[ \frac{d}{d x} \left(\frac{5 \ln{\left(\cos^{2}{\left(x \right)} - 1 \right)}}{4} - \frac{5 \ln{\left(\cos^{2}{\left(x \right)} \right)}}{4}\right) \]Differentiate the function with respect to x.✓ Proved
- \[ = \frac{5 \frac{d}{d x} \ln{\left(\cos^{2}{\left(x \right)} - 1 \right)}}{4} - \frac{5 \frac{d}{d x} \ln{\left(\cos^{2}{\left(x \right)} \right)}}{4} \]constant-multipleApply the constant multiple rule to each term.✓ Proved
- \[ = - \frac{5 \frac{d}{d x} \cos^{2}{\left(x \right)}}{4 \cos^{2}{\left(x \right)}} + \frac{5 \frac{d}{d x} \left(\cos^{2}{\left(x \right)} - 1\right)}{4 \left(\cos^{2}{\left(x \right)} - 1\right)} \]chainApply the chain rule to the logarithmic terms.✓ Proved
- \[ = - \frac{5 \frac{d}{d x} \cos{\left(x \right)}}{2 \cos{\left(x \right)}} + \frac{5 \cos{\left(x \right)} \frac{d}{d x} \cos{\left(x \right)}}{2 \left(\cos^{2}{\left(x \right)} - 1\right)} \]chainApply the chain rule to the squared terms.✓ Proved
- \[ = \frac{5 \sin{\left(x \right)}}{2 \cos{\left(x \right)}} - \frac{5 \sin{\left(x \right)} \cos{\left(x \right)}}{2 \left(\cos^{2}{\left(x \right)} - 1\right)} \]trig algebra algebra constant-multipleDifferentiate cos(x). Simplify the products and quotients. Simplify the second term by canceling cos(x). Simplify the coefficients.✓ Proved
- \[ = \frac{5 \tan{\left(x \right)}}{2} + \frac{5 \cos{\left(x \right)}}{2 \sin{\left(x \right)}} \]algebra algebraUse the identity cos(x)**2 - 1 = -sin(x)**2. Simplify the first term by canceling sin(x).✓ Proved
- \[ = \frac{5 \tan{\left(x \right)}}{2} + \frac{5}{2 \tan{\left(x \right)}} \]algebraRewrite cos(x)/sin(x) as 1/tan(x).✓ Proved
- \[ = \frac{5 \tan{\left(x \right)}}{2} + \frac{5 \cot{\left(x \right)}}{2} \]algebraFactor out the common term.✓ Proved
- \[ = \frac{5 \sin{\left(x \right)}}{2 \cos{\left(x \right)}} + \frac{5 \cos{\left(x \right)}}{2 \sin{\left(x \right)}} \]algebraRewrite cot(x) as cos(x)/sin(x).✓ Proved
- \[ = \frac{\frac{5 \sin^{2}{\left(x \right)}}{2} + \frac{5 \cos^{2}{\left(x \right)}}{2}}{\sin{\left(x \right)} \cos{\left(x \right)}} \]algebraCombine the terms under a common denominator.✓ Proved
- \[ = \frac{5}{2 \sin{\left(x \right)} \cos{\left(x \right)}} \]simplify algebraUse the Pythagorean identity sin(x)**2 + cos(x)**2 = 1. Distribute the 5/2.✓ Proved
- \[ = \frac{5}{\sin{\left(2 x \right)}} \]trigUse the double angle identity 2*sin(x)*cos(x) = sin(2*x).✓ Proved
- \[ = 5 \csc{\left(2 x \right)} \]simplifyRewrite 1/sin(2*x) as csc(2*x).✓ Proved
Answer \( \frac{5}{\sin{\left(2 x \right)}} \)
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.
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. The reviewers disagree about how one step is explained; every verdict is in the receipt.
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 undefined where cos(x)**2 - 1 = 0 undefined where cos(x) = 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(x)**2 - 1 = 0 undefined where cos(x) = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(x)**2 - 1 = 0 undefined where cos(x) = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(x)**2 - 1 = 0 undefined where cos(x) = 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(x)**2 - 1 = 0 undefined where cos(x) = 0 |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(x)**2 - 1 = 0 undefined where cos(x) = 0 |
| 9 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(x)**2 - 1 = 0 undefined where cos(x) = 0 tan has poles at odd multiples of pi/2 undefined where sin(x) = 0 |
| 10 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 tan has poles at odd multiples of pi/2 undefined where sin(x) = 0 |
| 11 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 tan has poles at odd multiples of pi/2 undefined where sin(x) = 0 undefined where tan(x) = 0 |
| 12 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 tan has poles at odd multiples of pi/2 undefined where tan(x) = 0 cot has poles at multiples of pi |
| 13 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 tan has poles at odd multiples of pi/2 cot has poles at multiples of pi undefined where cos(x) = 0 undefined where sin(x) = 0 |
| 14 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(x) = 0 undefined where sin(x) = 0 |
| 15 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(x) = 0 undefined where sin(x) = 0 |
| 16 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(x) = 0 undefined where sin(x) = 0 |
| 17 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where cos(x) = 0 undefined where sin(x) = 0 undefined where sin(2*x) = 0 |
| 18 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where sin(2*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(2*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
Every verdict on record (4)
qwen3.6:27b-mlx: pass 2026-10-03gpt-oss:20b: pass 2026-10-03qwen3.6:27b-mlx: fail (error) 2026-10-03 — Step 2 applies the linearity of the derivative (sum rule) to split the expression into two terms, but is labeled 'constant-multiple'. The constant-multiple rule only factors out constants from a single term; it does not split a sum. This violates the one-rule-per-step constraint and mislabels the operation.gpt-oss:20b: fail (error) 2026-10-03 — Step 8 incorrectly applies the constant‑multiple rule: the sign of the first term is wrong. The derivative of the first logarithmic term should yield +5/2*(cos(x)*sin(x)/(cos(x)**2-1)), not –5/2. This propagates an incorrect sign through the rest of the computation, leading to an incorrect final result.
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 2026-10-03 with SymPy 1.14.0.