Derivative of \( \displaystyle \frac{x}{x + 1} \)
Problem 2.12 · medium
Differentiate \( \displaystyle f(x) = \frac{x}{x + 1} \).
- \[ \frac{d}{d x} \frac{x}{x + 1} \]rewriteStart with the derivative of the function. Rewrite the fraction using a negative exponent.✓ Proved
- \[ = x \frac{d}{d x} \frac{1}{x + 1} + \frac{\frac{d}{d x} x}{x + 1} \]productApply the product rule.✓ Proved
- \[ = x \frac{d}{d x} \frac{1}{x + 1} + \frac{1}{x + 1} \]derivativeDifferentiate the first part of the product.✓ Proved
- \[ = - \frac{x \frac{d}{d x} \left(x + 1\right)}{\left(x + 1\right)^{2}} + \frac{1}{x + 1} \]chainApply the chain rule to the second part.✓ Proved
- \[ = - \frac{x}{\left(x + 1\right)^{2}} + \frac{1}{x + 1} \]derivative algebra rewrite algebraDifferentiate the inner function (x + 1). Simplify the expression. Rewrite the terms with positive exponents. Find a common denominator.✓ Proved
- \[ = \frac{1}{\left(x + 1\right)^{2}} \]algebra simplifyCombine the numerators. Simplify the final expression.✓ Proved
Answer \( \frac{1}{\left(x + 1\right)^{2}} \)
✓ 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.
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 + 1 = 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x + 1 = 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x + 1 = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x + 1 = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x + 1 = 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x + 1 = 0 |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x + 1 = 0 |
| 9 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x + 1 = 0 |
| 10 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x + 1 = 0 |
| 11 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x + 1 = 0 |
| answer | ✓ Proved | sympy 1.14.0 | final line against the stated answer: simplify(a - b) reduced to 0 undefined where x + 1 = 0 |
| answer, a second way | ✓ Proved | sympy 1.14.0 | SymPy differentiated f directly and got the stated answer |
Reviewers
deepseek-r1:70b: fail (style) — Step 1 lacks a rule label, which is required by the contract.gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly applies the product and chain rules with appropriate labels. Each step changes only one aspect of the expression, adhering to the single-rule constraint.
Every verdict on record (14)
qwen3.6:27b-mlx: pass 2026-09-20 — The solution correctly applies the product and chain rules with appropriate labels. Each step changes only one aspect of the expression, adhering to the single-rule constraint.gpt-oss:20b: pass 2026-09-20qwen3.6:27b-mlx: pass 2026-09-20gpt-oss:20b: pass 2026-09-20qwen3.6:27b-mlx: pass 2026-09-19deepseek-r1:70b: fail (style) 2026-09-19 — Step 1 lacks a rule label, which is required by the contract.gpt-oss:20b: pass 2026-09-19qwen3.6:27b-mlx: pass 2026-09-19deepseek-r1:70b: pass 2026-09-19gpt-oss:20b: pass 2026-09-19qwen3.6:27b-mlx: pass 2026-09-18deepseek-r1:70b: pass 2026-09-18gpt-oss:20b: pass 2026-09-18deepseek-r1:70b: pass 2026-09-16
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 2026-09-26 with SymPy 1.14.0.