∫Calc Practice

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 \( \displaystyle f(x) = \frac{5 \ln{\left(\cos^{2}{\left(x \right)} - 1 \right)}}{4} - \frac{5 \ln{\left(\cos^{2}{\left(x \right)} \right)}}{4} \).
  1. \[ \frac{d}{d x} \left(\frac{5 \ln{\left(\cos^{2}{\left(x \right)} - 1 \right)}}{4} - \frac{5 \ln{\left(\cos^{2}{\left(x \right)} \right)}}{4}\right) \]
    Differentiate the function with respect to x.✓ Proved
  2. \[ = \frac{5 \frac{d}{d x} \ln{\left(\cos^{2}{\left(x \right)} - 1 \right)}}{4} - \frac{5 \frac{d}{d x} \ln{\left(\cos^{2}{\left(x \right)} \right)}}{4} \]
    constant-multipleApply the constant multiple rule to each term.✓ Proved
  3. \[ = - \frac{5 \frac{d}{d x} \cos^{2}{\left(x \right)}}{4 \cos^{2}{\left(x \right)}} + \frac{5 \frac{d}{d x} \left(\cos^{2}{\left(x \right)} - 1\right)}{4 \left(\cos^{2}{\left(x \right)} - 1\right)} \]
    chainApply the chain rule to the logarithmic terms.✓ Proved
  4. \[ = - \frac{5 \frac{d}{d x} \cos{\left(x \right)}}{2 \cos{\left(x \right)}} + \frac{5 \cos{\left(x \right)} \frac{d}{d x} \cos{\left(x \right)}}{2 \left(\cos^{2}{\left(x \right)} - 1\right)} \]
    chainApply the chain rule to the squared terms.✓ Proved
  5. \[ = \frac{5 \sin{\left(x \right)}}{2 \cos{\left(x \right)}} - \frac{5 \sin{\left(x \right)} \cos{\left(x \right)}}{2 \left(\cos^{2}{\left(x \right)} - 1\right)} \]
    trig algebra algebra constant-multipleDifferentiate cos(x). Simplify the products and quotients. Simplify the second term by canceling cos(x). Simplify the coefficients.✓ Proved
  6. \[ = \frac{5 \tan{\left(x \right)}}{2} + \frac{5 \cos{\left(x \right)}}{2 \sin{\left(x \right)}} \]
    algebra algebraUse the identity cos(x)**2 - 1 = -sin(x)**2. Simplify the first term by canceling sin(x).✓ Proved
  7. \[ = \frac{5 \tan{\left(x \right)}}{2} + \frac{5}{2 \tan{\left(x \right)}} \]
    algebraRewrite cos(x)/sin(x) as 1/tan(x).✓ Proved
  8. \[ = \frac{5 \tan{\left(x \right)}}{2} + \frac{5 \cot{\left(x \right)}}{2} \]
    algebraFactor out the common term.✓ Proved
  9. \[ = \frac{5 \sin{\left(x \right)}}{2 \cos{\left(x \right)}} + \frac{5 \cos{\left(x \right)}}{2 \sin{\left(x \right)}} \]
    algebraRewrite cot(x) as cos(x)/sin(x).✓ Proved
  10. \[ = \frac{\frac{5 \sin^{2}{\left(x \right)}}{2} + \frac{5 \cos^{2}{\left(x \right)}}{2}}{\sin{\left(x \right)} \cos{\left(x \right)}} \]
    algebraCombine the terms under a common denominator.✓ Proved
  11. \[ = \frac{5}{2 \sin{\left(x \right)} \cos{\left(x \right)}} \]
    simplify algebraUse the Pythagorean identity sin(x)**2 + cos(x)**2 = 1. Distribute the 5/2.✓ Proved
  12. \[ = \frac{5}{\sin{\left(2 x \right)}} \]
    trigUse the double angle identity 2*sin(x)*cos(x) = sin(2*x).✓ Proved
  13. \[ = 5 \csc{\left(2 x \right)} \]
    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
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
undefined where cos(x)**2 - 1 = 0
undefined where cos(x) = 0
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where cos(x)**2 - 1 = 0
undefined where cos(x) = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where cos(x)**2 - 1 = 0
undefined where cos(x) = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where cos(x)**2 - 1 = 0
undefined where cos(x) = 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where cos(x)**2 - 1 = 0
undefined where cos(x) = 0
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where cos(x)**2 - 1 = 0
undefined where cos(x) = 0
9✓ Provedsympy 1.14.0simplify(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✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
tan has poles at odd multiples of pi/2
undefined where sin(x) = 0
11✓ Provedsympy 1.14.0simplify(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✓ Provedsympy 1.14.0simplify(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✓ Provedsympy 1.14.0simplify(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✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where cos(x) = 0
undefined where sin(x) = 0
15✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where cos(x) = 0
undefined where sin(x) = 0
16✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where cos(x) = 0
undefined where sin(x) = 0
17✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where cos(x) = 0
undefined where sin(x) = 0
undefined where sin(2*x) = 0
18✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where sin(2*x) = 0
csc has poles at multiples of pi
answer✓ Provedsympy 1.14.0final line against the stated answer: simplify(a - b) reduced to 0
undefined where sin(2*x) = 0
answer, a second way✓ Provedsympy 1.14.0SymPy differentiated f directly and got the stated answer

Reviewers

  • gpt-oss:20b: pass
  • qwen3.6:27b-mlx: pass
Every verdict on record (4)
  • qwen3.6:27b-mlx: pass 2026-10-03
  • gpt-oss:20b: pass 2026-10-03
  • qwen3.6:27b-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.