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Exponential and logarithmic equations

Problem 13.181 · easy

Solve \( \displaystyle 3^{x - 2} = 81 \) exactly.

Write 81 as 3^4 and set the exponents equal.
Reviewed
\[ \left. 3^{x - 2} - 81 \right|_{\substack{ x=6 }} = 0 \]
x = 6 satisfies the equation.✓ Proved

Answer \( x = 6 \)

✓ Nihil obstat Lines: 1 proved, 1 reviewed. The answer was also checked a second way, without looking at the solution. Reviewers found nothing wrong with the explanation. How we verify

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
answer, a second way	✓ Checked independently	sympy 1.14.0 + mpmath 1.3.0	Newton's method over [−20, 20] finds exactly the claimed real roots

Reviewers

gpt-oss:20b: pass
qwen3.6:27b-mlx: pass — The solution correctly identifies that 81 is 3^4 and sets the exponents equal, leading to the correct unique solution x=6. The verification step confirms the result.

Every verdict on record (6)

qwen3.6:27b-mlx: pass 2026-10-18 — The solution correctly identifies that 81 is 3^4 and sets the exponents equal, leading to the correct unique solution x=6. The verification step confirms the result.
gpt-oss:20b: pass 2026-10-18
qwen3.6:27b-mlx: pass 2026-10-16 — The solution correctly identifies that 81 is 3^4, sets the exponents equal to solve for x, and verifies the result. The logic is sound and complete.
gpt-oss:20b: pass 2026-10-16
qwen3.6:27b-mlx: pass 2026-10-14 — The solution correctly identifies that 81 is 3^4 and sets the exponents equal, leading to the correct exact solution. The verification step confirms the result.
gpt-oss:20b: pass 2026-10-14

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/log_exp_equations, checked 2026-10-18 with SymPy 1.14.0.