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Home›Calculus 1›Product rule›Problem 2.1428

Derivative of \( \displaystyle - \frac{x^{2}}{2} + x \left(x + 4\right) \ln{\left(x + 2 \right)} - 2 x + 4 \ln{\left(x + 2 \right)} \)

Problem 2.1428 · hard

Differentiate \( \displaystyle f(x) = - \frac{x^{2}}{2} + x \left(x + 4\right) \ln{\left(x + 2 \right)} - 2 x + 4 \ln{\left(x + 2 \right)} \).
  1. \[ \frac{d}{d x} \left(- \frac{x^{2}}{2} + x \left(x + 4\right) \ln{\left(x + 2 \right)} - 2 x + 4 \ln{\left(x + 2 \right)}\right) \]
    Start with the derivative of the function.✓ Proved
  2. \[ = \frac{d}{d x} \left(- 2 x\right) + \frac{d}{d x} \left(- \frac{x^{2}}{2}\right) + \frac{d}{d x} x \left(x + 4\right) \ln{\left(x + 2 \right)} + \frac{d}{d x} 4 \ln{\left(x + 2 \right)} \]
    sumApply the sum rule.✓ Proved
  3. \[ = \frac{d}{d x} \left(- 2 x\right) + \frac{d}{d x} \left(- \frac{x^{2}}{2}\right) + \frac{d}{d x} x \left(x + 4\right) \ln{\left(x + 2 \right)} + 4 \frac{d}{d x} \ln{\left(x + 2 \right)} \]
    constantApply the constant multiple rule to the last term.✓ Proved
  4. \[ = \frac{d}{d x} \left(- 2 x\right) + \frac{d}{d x} \left(- \frac{x^{2}}{2}\right) + \frac{d}{d x} x \left(x + 4\right) \ln{\left(x + 2 \right)} + \frac{4 \frac{d}{d x} \left(x + 2\right)}{x + 2} \]
    chainApply the chain rule to the log term.✓ Proved
  5. \[ = \frac{d}{d x} \left(- 2 x\right) + \frac{d}{d x} \left(- \frac{x^{2}}{2}\right) + \frac{d}{d x} x \left(x + 4\right) \ln{\left(x + 2 \right)} + \frac{4}{x + 2} \]
    derivative algebraDifferentiate the inner function x + 2. Simplify the last term.✓ Proved
  6. \[ = x \left(x + 4\right) \frac{d}{d x} \ln{\left(x + 2 \right)} + x \ln{\left(x + 2 \right)} \frac{d}{d x} \left(x + 4\right) + \left(x + 4\right) \ln{\left(x + 2 \right)} \frac{d}{d x} x + \frac{d}{d x} \left(- 2 x\right) + \frac{d}{d x} \left(- \frac{x^{2}}{2}\right) + \frac{4}{x + 2} \]
    productApply the product rule to the second term.✓ Proved
  7. \[ = x \left(x + 4\right) \frac{d}{d x} \ln{\left(x + 2 \right)} + x \ln{\left(x + 2 \right)} \frac{d}{d x} \left(x + 4\right) + \left(x + 4\right) \ln{\left(x + 2 \right)} + \frac{d}{d x} \left(- 2 x\right) + \frac{d}{d x} \left(- \frac{x^{2}}{2}\right) + \frac{4}{x + 2} \]
    derivativeDifferentiate x.✓ Proved
  8. \[ = x \left(x + 4\right) \frac{d}{d x} \ln{\left(x + 2 \right)} + x \ln{\left(x + 2 \right)} + \left(x + 4\right) \ln{\left(x + 2 \right)} + \frac{d}{d x} \left(- 2 x\right) + \frac{d}{d x} \left(- \frac{x^{2}}{2}\right) + \frac{4}{x + 2} \]
    derivativeDifferentiate x + 4.✓ Proved
  9. \[ = x \ln{\left(x + 2 \right)} + \frac{x \left(x + 4\right)}{x + 2} + \left(x + 4\right) \ln{\left(x + 2 \right)} + \frac{d}{d x} \left(- 2 x\right) + \frac{d}{d x} \left(- \frac{x^{2}}{2}\right) + \frac{4}{x + 2} \]
    derivative algebra algebraDifferentiate log(x + 2). Simplify the fraction. Remove unnecessary parentheses.✓ Proved
  10. \[ = x \ln{\left(x + 2 \right)} + \frac{x \left(x + 4\right)}{x + 2} + \left(x + 4\right) \ln{\left(x + 2 \right)} + \frac{d}{d x} \left(- \frac{x^{2}}{2}\right) - 2 + \frac{4}{x + 2} \]
    derivativeDifferentiate -2x.✓ Proved
  11. \[ = x \ln{\left(x + 2 \right)} - x + \frac{x \left(x + 4\right)}{x + 2} + \left(x + 4\right) \ln{\left(x + 2 \right)} - 2 + \frac{4}{x + 2} \]
    derivativeDifferentiate -x**2/2.✓ Proved
  12. \[ = - x + \left(2 x + 4\right) \ln{\left(x + 2 \right)} - 2 + \frac{x^{2} + 4 x}{x + 2} + \frac{4}{x + 2} \]
    algebraCombine log terms.✓ Proved
  13. \[ = - x + \left(2 x + 4\right) \ln{\left(x + 2 \right)} - 2 + \frac{x^{2} + 4 x + 4}{x + 2} \]
    algebraCombine the rational terms.✓ Proved
  14. \[ = \left(2 x + 4\right) \ln{\left(x + 2 \right)} \]
    algebra algebra simplifyFactor the numerator. Simplify the fraction. Combine like terms.✓ Proved
Answer \( 2 \left(x + 2\right) \ln{\left(x + 2 \right)} \)

✓ 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
log is undefined for non-positive arguments
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
9✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
10✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
11✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
12✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
13✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
14✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
15✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
16✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
17✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
undefined where x + 2 = 0
18✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
19✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
log is undefined for non-positive arguments
answer✓ Provedsympy 1.14.0final line against the stated answer: simplify(a - b) reduced to 0
log is undefined for non-positive arguments
answer, a second way✓ Provedsympy 1.14.0SymPy differentiated f directly and got the stated answer

Reviewers

  • gpt-oss:20b: fail (style) — Step 7 applies the product rule to the product of three factors in one go, violating the rule that each step may change only one thing.
  • qwen3.6:27b-mlx: fail (style) — [domain objection, downgraded to style] Step 7 applies the product rule to a product of three factors (x, x+4, and log(x+2)) in a single step, violating the contract that each step must change only one thing. The product rule is defined for two factors; applying it to three factors at once is a multi-rule application defect.
Every verdict on record (4)
  • qwen3.6:27b-mlx: fail (style) 2026-10-03 — [domain objection, downgraded to style] Step 7 applies the product rule to a product of three factors (x, x+4, and log(x+2)) in a single step, violating the contract that each step must change only one thing. The product rule is defined for two factors; applying it to three factors at once is a multi-rule application defect.
  • gpt-oss:20b: fail (style) 2026-10-03 — Step 7 applies the product rule to the product of three factors in one go, violating the rule that each step may change only one thing.
  • qwen3.6:27b-mlx: fail (style) 2026-10-03 — [domain objection, downgraded to style] Step 7 applies the product rule to a product of three factors (x, x+4, and log(x+2)) in a single step, violating the constraint that each step must change only one thing. The product rule is defined for two factors; applying it to three factors simultaneously is a defect in granularity.
  • 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.