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Derivative of \( \displaystyle \frac{x}{2} - \frac{\sin{\left(4 x \right)}}{8} \)

Problem 2.32 · hard

Differentiate \( \displaystyle f(x) = \frac{x}{2} - \frac{\sin{\left(4 x \right)}}{8} \).
  1. \[ \frac{d}{d x} \left(\frac{x}{2} - \frac{\sin{\left(4 x \right)}}{8}\right) \]
    derivativeStart with the derivative of the function.✓ Proved
  2. \[ = \frac{d}{d x} \frac{x}{2} - \frac{d}{d x} \frac{\sin{\left(4 x \right)}}{8} \]
    sumApply the sum rule for differentiation.✓ Proved
  3. \[ = \frac{d}{d x} \frac{x}{2} - \frac{\frac{d}{d x} \sin{\left(4 x \right)}}{8} \]
    constantPull out the constant factor from the second term.✓ Proved
  4. \[ = - \frac{\cos{\left(4 x \right)} \frac{d}{d x} 4 x}{8} + \frac{d}{d x} \frac{x}{2} \]
    chainApply the chain rule to the sine term.✓ Proved
  5. \[ = - \frac{\cos{\left(4 x \right)}}{2} + \frac{d}{d x} \frac{x}{2} \]
    derivative algebraDifferentiate the inner function 4*x. Simplify the constant coefficients.✓ Proved
  6. \[ = \frac{1}{2} - \frac{\cos{\left(4 x \right)}}{2} \]
    derivativeEvaluate the derivative of the first term.✓ Proved
  7. \[ = 0.5 - 0.5 \cos{\left(4 x \right)} \]
    simplifyFinal simplified form.✓ Proved
Answer \( \frac{1}{2} - \frac{\cos{\left(4 x \right)}}{2} \)

✓ 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
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
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: pass
Every verdict on record (12)
  • qwen3.6:27b-mlx: pass 2026-09-20
  • gpt-oss:20b: pass 2026-09-20
  • qwen3.6:27b-mlx: fail (style) 2026-09-20 — Step 3 is labeled 'constant' but pulls out a constant factor, which corresponds to the 'constant-multiple' rule in the fixed vocabulary. Step 2 is labeled 'sum' but applies the difference rule; while 'sum' is often used for linearity, the contract specifies 'sum' is acceptable for difference, so this is less severe, but the 'constant' vs 'constant-multiple' distinction is a clear vocabulary error.
  • gpt-oss:20b: pass 2026-09-20
  • qwen3.6:27b-mlx: fail (style) 2026-09-19 — Step 3 is labeled 'constant' but pulls out a constant factor, which should be labeled 'constant-multiple'. Step 7 is labeled 'derivative' but evaluates the derivative of x/2, which is correct, but the label 'derivative' is typically reserved for unfolding known derivatives; however, the main issue is Step 3's label. Actually, looking closer, Step 3 applies the constant multiple rule. The vocabulary has 'constant-multiple'. The step is labeled 'constant'. This is a labeling defect. Also, Step 2 is labeled 'sum' but it is a difference. The vocabulary says 'sum' covers difference. That is acceptable. Step 3 label 'constant' is incorrect; it should be 'constant-multiple'. Step 7 label 'derivative' is acceptable for evaluating d/dx(x/2). Step 6 label 'algebra' is acceptable. The primary defect is the label in Step 3.
  • 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: fail 2026-09-17 — Step 2 incorrectly labels the rule as a sum rule; it should be the linearity rule for a difference.
  • 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.