Limit of \( \displaystyle \frac{15 x - 7}{9 x + 4} \) as \( x \to \infty \)
Problem 1.107 · medium
Evaluate \( \displaystyle \lim_{x \to \infty} \frac{15 x - 7}{9 x + 4} \).
- \[ \lim_{x \to \infty}\left(\frac{15 x - 7}{9 x + 4}\right) \]limitStart with the original limit expression.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{15 - \frac{7}{x}}{9 + \frac{4}{x}}\right) \]algebra simplifyDivide both numerator and denominator by x. Simplify the fractions inside the limit.✓ Proved
- \[ = \lim_{x \to \infty}\left(15 - \frac{7}{x}\right) \left(\lim_{x \to \infty}\left(9 + \frac{4}{x}\right)\right)^{-1} \]limit-lawApply the quotient rule for limits.✓ Proved
- \[ = \lim_{x \to \infty} 15 \left(\lim_{x \to \infty} 9\right)^{-1} \]limitEvaluate the limits of the individual terms.✓ Proved
- \[ = \frac{5}{3} \]simplify simplifyPerform the division. Reduce the fraction to lowest terms.✓ Proved
Answer \( \frac{5}{3} \)
✓ 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.
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 9*x + 4 = 0 undefined where 9 + 4/x = 0 undefined where x = 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 9 + 4/x = 0 undefined where x = 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 9 + 4/x = 0 undefined where x = 0 undefined where Limit(9 + 4/x, x, oo, dir='-') = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where Limit(9 + 4/x, x, oo, dir='-') = 0 undefined where x = 0 undefined where Limit(9, x, oo, dir='-') = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where Limit(9, x, oo, dir='-') = 0 |
| 7 | ✓ 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
Every verdict on record (4)
qwen3.6:27b-mlx: pass 2026-10-03gpt-oss:20b: pass 2026-10-03qwen3.6:27b-mlx: pass 2026-10-03gpt-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.