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Definite integrals by substitution

Problem 4.457 · medium

Evaluate \( \displaystyle \int_{0}^{2} \frac{5 x}{x^{2} + 3}\, dx \).

Let u = x**2 + 3; then du = 2*x dx, which is in the integrand up to a constant.
Reviewed
\[ \left. x^{2} + 3 \right|_{\substack{ x=0 }} = 3 \]
The lower limit in u.✓ Proved
\[ \left. x^{2} + 3 \right|_{\substack{ x=2 }} = 7 \]
The upper limit in u.✓ Proved
\[ \int\limits_{3}^{7} \frac{5}{2 u}\, du = \ln{\left(\frac{49 \sqrt{21}}{27} \right)} \]
Integrate in u between the new limits; no back-substitution needed.✓ Proved

Answer \( \ln{\left(\frac{49 \sqrt{21}}{27} \right)} \)

Lines: 3 proved, 1 reviewed. 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

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	✓ Proved	sympy 1.14.0	simplify(lhs - rhs) reduced to 0
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	numerical quadrature of the original integral

Reviewers

gpt-oss:20b: pass
qwen3.6:27b-mlx: pass — The solution correctly applies u-substitution, transforms the limits accurately, and computes the definite integral. The final answer matches the stated answer.

Every verdict on record (4)

qwen3.6:27b-mlx: pass 2026-10-05 — The solution correctly applies u-substitution, transforms the limits accurately, and computes the definite integral. The final answer matches the stated answer.
gpt-oss:20b: pass 2026-10-05
qwen3.6:27b-mlx: fail (error) 2026-10-05 — The final answer is incorrect. The integral evaluates to (5/2) * ln(7/3), but the stated answer is ln(49*sqrt(21)/27), which is numerically different. The solution claims the integral equals the stated answer without showing the correct constant factor or logarithmic simplification.
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/definite_substitution, checked 2026-10-05 with SymPy 1.14.0.