∫Calc Practice

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

Problem 2.1482 · hard Beautiful

Differentiate \( \displaystyle f(x) = \ln{\left(\sqrt{\frac{x + 1}{1 - x}} \right)} \).
  1. \[ \frac{d}{d x} \ln{\left(\sqrt{\frac{x + 1}{1 - x}} \right)} \]
    derivativeStart with the derivative of the function.✓ Proved
  2. \[ = \frac{d}{d x} \ln{\left(\left(\frac{x + 1}{1 - x}\right)^{0.5} \right)} \]
    rewriteRewrite the square root as a power.✓ Proved
  3. \[ = \frac{d}{d x} 0.5 \ln{\left(\frac{x + 1}{1 - x} \right)} \]
    constant-multiplePull out the constant exponent.✓ Proved
  4. \[ = 0.5 \frac{d}{d x} \left(- \ln{\left(1 - x \right)} + \ln{\left(x + 1 \right)}\right) \]
    algebraUse the property of logarithms to split the quotient.✓ Proved
  5. \[ = - 0.5 \frac{d}{d x} \ln{\left(1 - x \right)} + 0.5 \frac{d}{d x} \ln{\left(x + 1 \right)} \]
    sumDistribute the derivative over the subtraction.✓ Proved
  6. \[ = - 0.5 \frac{d}{d x} \ln{\left(1 - x \right)} + \frac{0.5}{x + 1} \]
    derivativeDifferentiate the first term.✓ Proved
  7. \[ = \frac{0.5}{x + 1} + \frac{0.5}{1 - x} \]
    derivative algebraDifferentiate the second term using the chain rule. Simplify the signs.✓ Proved
  8. \[ = \frac{1.0}{\left(1 - x\right) \left(x + 1\right)} \]
    algebraFind a common denominator.✓ Proved
  9. \[ = \frac{1.0}{1 - x^{2}} \]
    simplifySimplify the numerator and denominator.✓ Proved
  10. \[ = \frac{1}{1 - x^{2}} \]
    simplifyFinal simplification.✓ Proved
Answer \( \frac{1}{1 - x^{2}} \)
Mind the domain. The answer is also defined on (-oo, -1) and (1, oo), 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
undefined where 1 - x = 0
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where 1 - x = 0
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where 1 - x = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 1 = 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 1 = 0
undefined where 1 - x = 0
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 1 - x = 0
undefined where x + 1 = 0
9✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 1 - x = 0
undefined where x + 1 = 0
10✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 1 - x = 0
undefined where x + 1 = 0
undefined where 1 - x**2 = 0
11✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 1 - x**2 = 0
answer✓ Provedsympy 1.14.0final line against the stated answer: simplify(a - b) reduced to 0
undefined where 1 - x**2 = 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: fail (error) — Step 3 incorrectly labels the application of the logarithm power rule (log(a^b) = b*log(a)) as 'constant-multiple'. The 'constant-multiple' rule applies to the derivative operator (d/dx [c*f] = c*d/dx f), not to the algebraic manipulation of the function inside the derivative. This is a mislabeling of an algebraic/logarithmic simplification step.
Every verdict on record (4)
  • qwen3.6:27b-mlx: fail (error) 2026-10-04 — Step 3 incorrectly labels the application of the logarithm power rule (log(a^b) = b*log(a)) as 'constant-multiple'. The 'constant-multiple' rule applies to the derivative operator (d/dx [c*f] = c*d/dx f), not to the algebraic manipulation of the function inside the derivative. This is a mislabeling of an algebraic/logarithmic simplification step.
  • gpt-oss:20b: pass 2026-10-04
  • qwen3.6:27b-mlx: pass 2026-10-04
  • gpt-oss:20b: pass 2026-10-04

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 2026-10-04 with SymPy 1.14.0.