Derivative of \( \displaystyle 4 x \left(x + 1\right) \)
Problem 2.78 · easy
Differentiate \( \displaystyle f(x) = 4 x \left(x + 1\right) \).
- \[ \frac{d}{d x} 4 x \left(x + 1\right) \]derivativeStart with the derivative of the function.✓ Proved
- \[ = 4 \frac{d}{d x} x \left(x + 1\right) \]constant-multiplePull out the constant factor 4.✓ Proved
- \[ = 4 x \frac{d}{d x} \left(x + 1\right) + 4 \left(x + 1\right) \frac{d}{d x} x \]productApply the product rule to x*(x + 1).✓ Proved
- \[ = 4 x \frac{d}{d x} \left(x + 1\right) + 4 x + 4 \]derivative algebraDifferentiate x and (x + 1). Simplify the expression inside the parentheses.✓ Proved
- \[ = 4 x \left(\frac{d}{d x} 1 + \frac{d}{d x} x\right) + 4 x + 4 \]sumApply the sum rule to the derivative of (x + 1).✓ Proved
- \[ = 8 x + 4 \]derivative algebra algebra algebraDifferentiate x and the constant 1. Simplify the expression inside the parentheses. Combine like terms. Distribute the 4.✓ Proved
Answer \( 8 x + 4 \)
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. The reviewers disagree about how one step is explained; every verdict is in the receipt.
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 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 9 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 10 | ✓ 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 differentiated f directly and got the stated answer |
Reviewers
gpt-oss:20b: passdeepseek-r1:70b: passqwen3.6:27b-mlx: fail (error) — Step 4 claims to differentiate both x and (x+1) using the 'derivative' rule, but it only differentiates x, leaving Derivative(x+1, x) unevaluated. This violates the single-change constraint and mislabels the step, as the differentiation of the second term is deferred to Step 6.
Every verdict on record (12)
qwen3.6:27b-mlx: fail (error) 2026-09-20 — Step 4 claims to differentiate both x and (x+1) using the 'derivative' rule, but it only differentiates x, leaving Derivative(x+1, x) unevaluated. This violates the single-change constraint and mislabels the step, as the differentiation of the second term is deferred to Step 6.gpt-oss:20b: pass 2026-09-20qwen3.6:27b-mlx: fail (error) 2026-09-20 — Step 4 claims to apply the 'derivative' rule but performs two distinct operations: differentiating x to 1 and expanding the derivative of the sum (x+1) into separate terms. This violates the single-rule-per-step constraint. Additionally, Step 6 applies the 'sum' rule to Derivative(x+1, x), but the sum rule applies to the function inside the derivative, not the derivative operator itself; the step should have been labeled 'derivative' (unfolding d/dx of a sum) or the expansion should have happened in step 4.gpt-oss:20b: pass 2026-09-20qwen3.6:27b-mlx: fail (error) 2026-09-19 — Step 4 claims to apply the 'derivative' rule but performs two distinct operations: differentiating x to 1 and expanding the derivative of the sum (x+1) into separate terms. This violates the single-rule-per-step constraint.deepseek-r1:70b: pass 2026-09-19gpt-oss:20b: pass 2026-09-19qwen3.6:27b-mlx: pass 2026-09-19deepseek-r1:70b: pass 2026-09-19gpt-oss:20b: pass 2026-09-19gpt-oss:20b: pass 2026-09-17deepseek-r1:70b: pass 2026-09-17
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.