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Derivative of \( \displaystyle - x + \tan{\left(x \right)} \)

Problem 2.11 · medium

Differentiate \( \displaystyle f(x) = - x + \tan{\left(x \right)} \).
  1. \[ \frac{d}{d x} \left(- x + \tan{\left(x \right)}\right) \]
    derivativeStart with the derivative of the function.✓ Proved
  2. \[ = \frac{d}{d x} \left(- x\right) + \frac{d}{d x} \tan{\left(x \right)} \]
    sumApply the sum rule.✓ Proved
  3. \[ = - \frac{d}{d x} x + \frac{d}{d x} \tan{\left(x \right)} \]
    constantApply the constant multiple rule to the first term.✓ Proved
  4. \[ = \frac{d}{d x} \tan{\left(x \right)} - 1 \]
    derivativeDifferentiate the first term.✓ Proved
  5. \[ = \sec^{2}{\left(x \right)} - 1 \]
    trig algebraDifferentiate the tangent function. Rearrange the terms.✓ Proved
Answer \( \tan^{2}{\left(x \right)} \)
Mind the domain. The answer is also defined at points where f(x) is not. Substituting there gives a number that is not a slope of f.

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
tan has poles at odd multiples of pi/2
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
tan has poles at odd multiples of pi/2
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
tan has poles at odd multiples of pi/2
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
tan has poles at odd multiples of pi/2
sec has poles at odd multiples of pi/2
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
sec has poles at odd multiples of pi/2
answer✓ Provedsympy 1.14.0final line against the stated answer: simplify(a - b) reduced to 0
tan has poles at odd multiples of pi/2
answer, a second way✓ Provedsympy 1.14.0SymPy differentiated f directly and got the stated answer

Reviewers

  • deepseek-r1:70b: pass
  • gpt-oss:20b: pass
  • qwen3.6:27b-mlx: pass
Every verdict on record (14)
  • qwen3.6:27b-mlx: pass 2026-09-20
  • gpt-oss:20b: pass 2026-09-20
  • qwen3.6:27b-mlx: pass 2026-09-20
  • gpt-oss:20b: fail (style) 2026-09-20 — Step 6 rearranges -1+sec(x)**2 to sec(x)**2-1 using the trigonometric identity sec^2(x)-1=tan^2(x). The label "algebra" is inappropriate; it should be "trig" (or a specific identity rule).
  • qwen3.6:27b-mlx: pass 2026-09-19
  • deepseek-r1:70b: pass 2026-09-19
  • gpt-oss:20b: fail (error) 2026-09-19 — The solution stops at sec(x)**2 - 1, but the required final answer is tan(x)**2. The identity sec^2(x)-1 = tan^2(x) is missing, so the solution is incomplete and mathematically incorrect.
  • qwen3.6:27b-mlx: pass 2026-09-19
  • deepseek-r1:70b: pass 2026-09-19
  • gpt-oss:20b: pass 2026-09-19
  • qwen3.6:27b-mlx: pass 2026-09-18
  • deepseek-r1:70b: pass 2026-09-18
  • gpt-oss:20b: pass 2026-09-18
  • deepseek-r1:70b: pass 2026-09-16

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.