Trigonometric substitution
Problem 4.423 · hard
Evaluate \( \displaystyle \int \frac{1}{x^{2} \sqrt{x^{2} + 16}}\, dx \).
- The radical suggests x = 4 tan θ.Reviewed
- \[ 16 \tan^{2}{\left(\theta \right)} + 16 = 16 \sec^{2}{\left(\theta \right)} \]The identity that clears the square root.✓ Proved
- Rewrite the integral in θ, integrate, then use a right triangle to express the trig functions of θ in terms of x.Reviewed
- \[ \frac{d}{d x} \left(- \frac{\sqrt{x^{2} + 16}}{16 x}\right) = \frac{1}{x^{2} \sqrt{x^{2} + 16}} \]Differentiating the answer returns the integrand.✓ Proved
Answer \( - \frac{\sqrt{x^{2} + 16}}{16 x} + C \)
✓ Nihil obstat Lines: 2 proved, 2 reviewed. The answer was also checked a second way, without looking at the solution. Reviewers found nothing wrong with the explanation.
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | Reviewed | reviewers: gpt-oss:20b, qwen3.6:27b-mlx | a sentence; read, not computed |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 3 | Reviewed | reviewers: gpt-oss:20b, qwen3.6:27b-mlx | a sentence; read, not computed |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| answer, a second way | ✓ Checked independently | sympy 1.14.0 + mpmath 1.3.0 | F(b) − F(a) matches numerical quadrature of the integrand on a sub-interval of the domain |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly identifies the trigonometric substitution and verifies the result via differentiation. Although the intermediate integration steps are omitted, the provided steps are logically sound and the final answer is correct.
Every verdict on record (4)
qwen3.6:27b-mlx: pass 2026-10-05 — The solution correctly identifies the trigonometric substitution and verifies the result via differentiation. Although the intermediate integration steps are omitted, the provided steps are logically sound and the final answer is correct.gpt-oss:20b: pass 2026-10-05qwen3.6:27b-mlx: pass 2026-10-05 — The solution correctly identifies the trigonometric substitution and verifies the result by differentiation. Although the intermediate integration steps are omitted, the stated method and final verification are sound.gpt-oss:20b: pass 2026-10-05
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
generator:structured/trig_substitution, checked 2026-10-05 with SymPy 1.14.0.