∫Calc Practice

Derivative of \( \displaystyle \frac{\ln{\left(\sin{\left(2 x \right)} - 1 \right)}}{4} - \frac{\ln{\left(\sin{\left(2 x \right)} + 1 \right)}}{4} \)

Problem 2.1308 · hard

Differentiate \( \displaystyle f(x) = \frac{\ln{\left(\sin{\left(2 x \right)} - 1 \right)}}{4} - \frac{\ln{\left(\sin{\left(2 x \right)} + 1 \right)}}{4} \).
  1. \[ \frac{d}{d x} \left(\frac{\ln{\left(\sin{\left(2 x \right)} - 1 \right)}}{4} - \frac{\ln{\left(\sin{\left(2 x \right)} + 1 \right)}}{4}\right) \]
    Start with the derivative of the function.✓ Proved
  2. \[ = \frac{\frac{d}{d x} \ln{\left(\sin{\left(2 x \right)} - 1 \right)}}{4} - \frac{\frac{d}{d x} \ln{\left(\sin{\left(2 x \right)} + 1 \right)}}{4} \]
    constant-multiple algebraApply the constant multiple rule to each term. Factor out the common constant 1/4.✓ Proved
  3. \[ = - \frac{\frac{d}{d x} \left(\sin{\left(2 x \right)} + 1\right)}{4 \left(\sin{\left(2 x \right)} + 1\right)} + \frac{\frac{d}{d x} \left(\sin{\left(2 x \right)} - 1\right)}{4 \left(\sin{\left(2 x \right)} - 1\right)} \]
    chainApply the chain rule to the logarithmic terms.✓ Proved
  4. \[ = - \frac{\frac{d}{d x} 1 + \frac{d}{d x} \sin{\left(2 x \right)}}{4 \left(\sin{\left(2 x \right)} + 1\right)} + \frac{- \frac{d}{d x} 1 + \frac{d}{d x} \sin{\left(2 x \right)}}{4 \left(\sin{\left(2 x \right)} - 1\right)} \]
    sumDifferentiate the terms inside the parentheses.✓ Proved
  5. \[ = - \frac{\frac{d}{d x} \sin{\left(2 x \right)}}{4 \left(\sin{\left(2 x \right)} + 1\right)} + \frac{\frac{d}{d x} \sin{\left(2 x \right)}}{4 \left(\sin{\left(2 x \right)} - 1\right)} \]
    constant algebraThe derivative of a constant is zero. Simplify the expression by removing zero terms.✓ Proved
  6. \[ = \frac{\left(- \frac{1}{\sin{\left(2 x \right)} + 1} + \frac{1}{\sin{\left(2 x \right)} - 1}\right) \frac{d}{d x} \sin{\left(2 x \right)}}{4} \]
    algebraFactor out the common derivative term.✓ Proved
  7. \[ = \frac{\left(- \frac{1}{\sin{\left(2 x \right)} + 1} + \frac{1}{\sin{\left(2 x \right)} - 1}\right) \cos{\left(2 x \right)}}{2} \]
    trig algebraDifferentiate the sine term using the chain rule. Simplify the constant coefficient.✓ Proved
  8. \[ = \frac{\cos{\left(2 x \right)}}{\left(\sin{\left(2 x \right)} - 1\right) \left(\sin{\left(2 x \right)} + 1\right)} \]
    algebraCombine the fractions using a common denominator.✓ Proved
  9. \[ = \frac{\cos{\left(2 x \right)}}{\sin^{2}{\left(2 x \right)} - 1} \]
    algebra algebraSimplify the numerator and the denominator. Simplify the expression by canceling the 2 and the 1/2.✓ Proved
  10. \[ = - \frac{\cos{\left(2 x \right)}}{1 - \sin^{2}{\left(2 x \right)}} \]
    algebraRewrite the denominator to use a trigonometric identity.✓ Proved
  11. \[ = - \frac{1}{\cos{\left(2 x \right)}} \]
    rewrite simplifyUse the Pythagorean identity 1 - sin^2(u) = cos^2(u). Simplify the fraction by canceling cos(2*x).✓ Proved
  12. \[ = - \sec{\left(2 x \right)} \]
    rewriteRewrite 1/cos(u) as sec(u).✓ Proved
Answer \( - \frac{1}{\cos{\left(2 x \right)}} \)
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.

✓ 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
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
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where sin(2*x) + 1 = 0
undefined where sin(2*x) - 1 = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where sin(2*x) + 1 = 0
undefined where sin(2*x) - 1 = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where sin(2*x) + 1 = 0
undefined where sin(2*x) - 1 = 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where sin(2*x) + 1 = 0
undefined where sin(2*x) - 1 = 0
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where sin(2*x) + 1 = 0
undefined where sin(2*x) - 1 = 0
9✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where sin(2*x) + 1 = 0
undefined where sin(2*x) - 1 = 0
10✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where sin(2*x) + 1 = 0
undefined where sin(2*x) - 1 = 0
11✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where sin(2*x) + 1 = 0
undefined where sin(2*x) - 1 = 0
12✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where sin(2*x) + 1 = 0
undefined where sin(2*x) - 1 = 0
undefined where sin(2*x)**2 - 1 = 0
13✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where sin(2*x)**2 - 1 = 0
14✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where sin(2*x)**2 - 1 = 0
undefined where 1 - sin(2*x)**2 = 0
15✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 1 - sin(2*x)**2 = 0
undefined where cos(2*x) = 0
16✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where cos(2*x) = 0
17✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where cos(2*x) = 0
sec has poles at odd multiples of pi/2
answer✓ Provedsympy 1.14.0final line against the stated answer: simplify(a - b) reduced to 0
undefined where cos(2*x) = 0
answer, a second way✓ Provedsympy 1.14.0SymPy differentiated f directly and got the stated answer

Reviewers

  • gpt-oss:20b: fail (style) — Step 9 incorrectly labels the application of the chain rule as "trig"; the correct label should be "chain" (the derivative of sin(2*x) is 2*cos(2*x), which is a chain rule application).
  • qwen3.6:27b-mlx: pass — 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.
Every verdict on record (4)
  • qwen3.6:27b-mlx: pass 2026-10-03 — 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: fail (style) 2026-10-03 — Step 9 incorrectly labels the application of the chain rule as "trig"; the correct label should be "chain" (the derivative of sin(2*x) is 2*cos(2*x), which is a chain rule application).
  • qwen3.6:27b-mlx: pass 2026-09-30
  • gpt-oss:20b: fail (error) 2026-09-30 — Step 4 applies the chain rule to both logarithmic terms in a single line, violating the rule‑by‑rule granularity requirement. Each application of the chain rule should be a separate step with its own label.

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.