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Derivative of \( \displaystyle - 9 x^{3} - 9 x^{2} - 3 x \)

Problem 2.1842 · medium

Differentiate \( \displaystyle f(x) = - 9 x^{3} - 9 x^{2} - 3 x \).
  1. \[ \frac{d}{d x} \left(- 9 x^{3} - 9 x^{2} - 3 x\right) \]
    derivativeStart with the derivative of the function.✓ Proved
  2. \[ = \frac{d}{d x} \left(- 3 x\right) + \frac{d}{d x} \left(- 9 x^{2}\right) + \frac{d}{d x} \left(- 9 x^{3}\right) \]
    sumApply the sum rule to differentiate each term separately.✓ Proved
  3. \[ = \frac{d}{d x} \left(- 3 x\right) + \frac{d}{d x} \left(- 9 x^{2}\right) - 9 \frac{d}{d x} x^{3} \]
    constant-multipleFactor out the constant -9 from the first term.✓ Proved
  4. \[ = \frac{d}{d x} \left(- 3 x\right) - 9 \frac{d}{d x} x^{2} - 9 \frac{d}{d x} x^{3} \]
    constant-multipleFactor out the constant -9 from the second term.✓ Proved
  5. \[ = - 3 \frac{d}{d x} x - 9 \frac{d}{d x} x^{2} - 9 \frac{d}{d x} x^{3} \]
    constant-multipleFactor out the constant -3 from the third term.✓ Proved
  6. \[ = - 27 x^{2} - 18 x - 3 \]
    power algebraApply the power rule to each term. Multiply the constants through the parentheses.✓ Proved
Answer \( - 3 \left(3 x + 1\right)^{2} \)

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
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
  • qwen3.6:27b-mlx: pass
Every verdict on record (4)
  • qwen3.6:27b-mlx: pass 2026-10-08
  • gpt-oss:20b: pass 2026-10-08
  • gpt-oss:20b: fail (error) 2026-10-08 — Step 6 applies the power rule to all three terms in one line, violating the rule that each step must change only one thing. The correct approach would apply the power rule separately to each term in distinct steps.
  • qwen3.6:27b-mlx: pass 2026-10-08

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 2026-10-08 with SymPy 1.14.0.