∫Calc Practice
Home›Calculus 1›Chain rule›Problem 2.1424

Derivative of \( \displaystyle \ln{\left(\frac{3 x}{\sqrt{9 x^{2} + 1}} \right)} \)

Problem 2.1424 · hard

Differentiate \( \displaystyle f(x) = \ln{\left(\frac{3 x}{\sqrt{9 x^{2} + 1}} \right)} \).
  1. \[ \frac{d}{d x} \ln{\left(\frac{3 x}{\sqrt{9 x^{2} + 1}} \right)} \]
    derivativeStart with the derivative of the function.✓ Proved
  2. \[ = \frac{d}{d x} \left(\ln{\left(3 x \right)} - \ln{\left(\sqrt{9 x^{2} + 1} \right)}\right) \]
    simplify simplifyUse logarithm properties to split the quotient. Rewrite the square root as a power.✓ Proved
  3. \[ = \frac{d}{d x} \left(\ln{\left(3 x \right)} - \frac{\ln{\left(9 x^{2} + 1 \right)}}{2}\right) \]
    algebraMove the exponent to the front.✓ Proved
  4. \[ = \frac{d}{d x} \ln{\left(3 x \right)} - \frac{d}{d x} \frac{\ln{\left(9 x^{2} + 1 \right)}}{2} \]
    sumApply the sum rule for derivatives.✓ Proved
  5. \[ = \frac{d}{d x} \ln{\left(3 x \right)} - \frac{\frac{d}{d x} \ln{\left(9 x^{2} + 1 \right)}}{2} \]
    constant-multiplePull out the constant factor.✓ Proved
  6. \[ = \frac{d}{d x} \ln{\left(3 x \right)} - \frac{\frac{d}{d x} \left(9 x^{2} + 1\right)}{2 \left(9 x^{2} + 1\right)} \]
    logarithmicApply the derivative rule for the natural logarithm.✓ Proved
  7. \[ = - \frac{9 x}{9 x^{2} + 1} + \frac{d}{d x} \ln{\left(3 x \right)} \]
    derivativeDifferentiate the inner polynomial.✓ Proved
  8. \[ = - \frac{9 x}{9 x^{2} + 1} + \frac{1}{x} \]
    logarithmic algebraDifferentiate the first term. Simplify the second term.✓ Proved
  9. \[ = \frac{1}{x \left(9 x^{2} + 1\right)} \]
    algebra algebra simplifyFind a common denominator. Simplify the numerator. Final simplification.✓ Proved
Answer \( \frac{1}{9 x^{3} + x} \)
Mind the domain. The answer is also defined on (-oo, 0), 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 9*x**2 + 1 = 0
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
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
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where 9*x**2 + 1 = 0
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where 9*x**2 + 1 = 0
9✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where 9*x**2 + 1 = 0
undefined where x = 0
10✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 9*x**2 + 1 = 0
undefined where x = 0
11✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 9*x**2 + 1 = 0
undefined where x = 0
12✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 9*x**2 + 1 = 0
undefined where x = 0
13✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 9*x**2 + 1 = 0
undefined where x = 0
answer✓ Provedsympy 1.14.0final line against the stated answer: simplify(a - b) reduced to 0
undefined where 9*x**3 + x = 0
answer, a second way✓ Provedsympy 1.14.0SymPy differentiated f directly and got the stated answer

Reviewers

  • gpt-oss:20b: fail (error) — Step 7 applies two rules at once: the logarithmic derivative rule and the inner derivative. This violates the one‑rule‑per‑step requirement. The same occurs in step 9.
  • qwen3.6:27b-mlx: pass — The solution correctly applies differentiation rules and algebraic simplifications. Each step modifies only one aspect of the expression and uses valid labels from the fixed vocabulary.
Every verdict on record (4)
  • qwen3.6:27b-mlx: pass 2026-10-03 — The solution correctly applies differentiation rules and algebraic simplifications. Each step modifies only one aspect of the expression and uses valid labels from the fixed vocabulary.
  • gpt-oss:20b: fail (error) 2026-10-03 — Step 7 applies two rules at once: the logarithmic derivative rule and the inner derivative. This violates the one‑rule‑per‑step requirement. The same occurs in step 9.
  • qwen3.6:27b-mlx: pass 2026-10-03
  • gpt-oss:20b: pass 2026-10-03

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.