Limit of \( \displaystyle \frac{1 - \cos{\left(3 x \right)}}{9 x^{2}} \) as \( x \to 0 \)
Problem 1.26 · medium Video
Evaluate \( \displaystyle \lim_{x \to 0} \frac{1 - \cos{\left(3 x \right)}}{9 x^{2}} \).
- \[ \lim_{x \to 0^+}\left(\frac{1 - \cos{\left(3 x \right)}}{9 x^{2}}\right) \]limitStart with the limit of the original function.✓ Proved
- \[ = \lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(1 - \cos{\left(3 x \right)}\right)}{\frac{d}{d x} 9 x^{2}}\right) \]lhopitalApply L'Hôpital's rule because the limit is an indeterminate form 0/0.✓ Proved
- \[ = \lim_{x \to 0^+}\left(\frac{\sin{\left(3 x \right)}}{6 x}\right) \]simplify algebra rewriteDifferentiate the numerator and denominator. Simplify the fraction by dividing the constants. Rewrite the expression to use the standard trigonometric limit.✓ Proved
- \[ = \frac{\lim_{x \to 0^+}\left(\frac{\sin{\left(3 x \right)}}{3 x}\right)}{2} \]limit-lawApply the limit-law for the product of two functions.✓ Proved
- \[ = \frac{1}{2} \]trig-limit simplifyUse the standard limit result sin(u)/u -> 1 as u -> 0. Final evaluation.✓ Proved
Answer \( \frac{1}{2} \)
✓ Nihil obstat 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. Reviewers found nothing wrong with the explanation. How we verify
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | ✓ Proved | sympy 1.14.0 | line 1 is the problem as stated |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 undefined where Derivative(9*x**2, x) = 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where Derivative(9*x**2, x) = 0 undefined where x = 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| answer | ✓ Proved | sympy 1.14.0 | final line against the stated answer: simplify(a - b) reduced to 0 |
| answer, a second way | ✓ Proved | sympy 1.14.0 | SymPy took the limit from both sides and got the stated value |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly applies L'Hôpital's rule, simplifies the resulting expression, and uses the standard trigonometric limit. Each step changes only one aspect of the expression and uses valid labels from the fixed vocabulary.
Every verdict on record (6)
qwen3.6:27b-mlx: pass 2026-10-03 — The solution correctly applies L'Hôpital's rule, simplifies the resulting expression, and uses the standard trigonometric limit. Each step changes only one aspect of the expression and uses valid labels from the fixed vocabulary.gpt-oss:20b: pass 2026-10-03qwen3.6:27b-mlx: pass 2026-09-29 — The solution correctly applies L'Hôpital's rule, simplifies the resulting expression, and uses the standard trigonometric limit. Each step changes only one aspect of the expression and uses valid labels from the fixed vocabulary.gpt-oss:20b: pass 2026-09-29qwen3.6:27b-mlx: pass 2026-09-29gpt-oss:20b: pass 2026-09-29
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.