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Derivative of \( \displaystyle 4 x \left(x + 1\right) \)

Problem 2.78 · easy

Differentiate \( \displaystyle f(x) = 4 x \left(x + 1\right) \).
  1. \[ \frac{d}{d x} 4 x \left(x + 1\right) \]
    derivativeStart with the derivative of the function.✓ Proved
  2. \[ = 4 \frac{d}{d x} x \left(x + 1\right) \]
    constant-multiplePull out the constant factor 4.✓ Proved
  3. \[ = 4 x \frac{d}{d x} \left(x + 1\right) + 4 \left(x + 1\right) \frac{d}{d x} x \]
    productApply the product rule to x*(x + 1).✓ Proved
  4. \[ = 4 x \frac{d}{d x} \left(x + 1\right) + 4 x + 4 \]
    derivative algebraDifferentiate x and (x + 1). Simplify the expression inside the parentheses.✓ Proved
  5. \[ = 4 x \left(\frac{d}{d x} 1 + \frac{d}{d x} x\right) + 4 x + 4 \]
    sumApply the sum rule to the derivative of (x + 1).✓ Proved
  6. \[ = 8 x + 4 \]
    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
LineStatusChecked byDetail
1✓ Provedsympy 1.14.0line 1 is the problem as stated
2✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
9✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
10✓ 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 differentiated f directly and got the stated answer

Reviewers

  • gpt-oss:20b: pass
  • deepseek-r1:70b: pass
  • qwen3.6:27b-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 2026-09-20
  • qwen3.6:27b-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 2026-09-20
  • qwen3.6:27b-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 2026-09-19
  • gpt-oss:20b: pass 2026-09-19
  • qwen3.6:27b-mlx: pass 2026-09-19
  • deepseek-r1:70b: pass 2026-09-19
  • gpt-oss:20b: pass 2026-09-19
  • gpt-oss:20b: pass 2026-09-17
  • deepseek-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.