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Home›Calculus 1›Limits at infinity›Problem 1.107

Limit of \( \displaystyle \frac{15 x - 7}{9 x + 4} \) as \( x \to \infty \)

Problem 1.107 · medium

Evaluate \( \displaystyle \lim_{x \to \infty} \frac{15 x - 7}{9 x + 4} \).
  1. \[ \lim_{x \to \infty}\left(\frac{15 x - 7}{9 x + 4}\right) \]
    limitStart with the original limit expression.✓ Proved
  2. \[ = \lim_{x \to \infty}\left(\frac{15 - \frac{7}{x}}{9 + \frac{4}{x}}\right) \]
    algebra simplifyDivide both numerator and denominator by x. Simplify the fractions inside the limit.✓ Proved
  3. \[ = \lim_{x \to \infty}\left(15 - \frac{7}{x}\right) \left(\lim_{x \to \infty}\left(9 + \frac{4}{x}\right)\right)^{-1} \]
    limit-lawApply the quotient rule for limits.✓ Proved
  4. \[ = \lim_{x \to \infty} 15 \left(\lim_{x \to \infty} 9\right)^{-1} \]
    limitEvaluate the limits of the individual terms.✓ Proved
  5. \[ = \frac{5}{3} \]
    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
LineStatusChecked byDetail
1✓ Provedsympy 1.14.0line 1 is the problem as stated
2✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 9*x + 4 = 0
undefined where 9 + 4/x = 0
undefined where x = 0
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 9 + 4/x = 0
undefined where x = 0
4✓ Provedsympy 1.14.0simplify(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✓ Provedsympy 1.14.0simplify(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✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where Limit(9, x, oo, dir='-') = 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
answer✓ Provedsympy 1.14.0final line against the stated answer: simplify(a - b) reduced to 0
answer, a second way✓ Provedsympy 1.14.0SymPy took the limit from both sides and got the stated value

Reviewers

  • gpt-oss:20b: pass
  • qwen3.6:27b-mlx: pass
Every verdict on record (4)
  • qwen3.6:27b-mlx: pass 2026-10-03
  • gpt-oss:20b: pass 2026-10-03
  • qwen3.6:27b-mlx: pass 2026-10-03
  • gpt-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.