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Derivative of \( \displaystyle 3 \sqrt{\left(x + 2\right)^{2} + 1} \)

Problem 2.155 · medium

Differentiate \( \displaystyle f(x) = 3 \sqrt{\left(x + 2\right)^{2} + 1} \).
  1. \[ \frac{d}{d x} 3 \sqrt{\left(x + 2\right)^{2} + 1} \]
    derivativeStart with the derivative of the function.✓ Proved
  2. \[ = 3 \frac{d}{d x} \sqrt{\left(x + 2\right)^{2} + 1} \]
    constant-multiple rewritePull the constant factor out of the derivative. Rewrite the square root as a fractional power.✓ Proved
  3. \[ = \frac{3 \frac{d}{d x} \left(\left(x + 2\right)^{2} + 1\right)}{2 \sqrt{\left(x + 2\right)^{2} + 1}} \]
    chainApply the chain rule.✓ Proved
  4. \[ = \frac{3 \left(2 x + 4\right)}{2 \sqrt{\left(x + 2\right)^{2} + 1}} \]
    derivative simplifyDifferentiate the inner function. Simplify the derivative of the inner term.✓ Proved
  5. \[ = \frac{3 x + 6}{\sqrt{\left(x + 2\right)^{2} + 1}} \]
    algebra rewriteCancel the 2 and the 1/2. Rewrite the negative exponent as a fraction.✓ Proved
Answer \( \frac{3 \left(x + 2\right)}{\sqrt{\left(x + 2\right)^{2} + 1}} \)

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
undefined where (x + 2)**2 + 1 = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where (x + 2)**2 + 1 = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where (x + 2)**2 + 1 = 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where (x + 2)**2 + 1 = 0
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where (x + 2)**2 + 1 = 0
answer✓ Provedsympy 1.14.0final line against the stated answer: simplify(a - b) reduced to 0
undefined where (x + 2)**2 + 1 = 0
answer, a second way✓ Provedsympy 1.14.0SymPy differentiated f directly and got the stated answer

Reviewers

  • gpt-oss:20b: fail (error) — Step 4 applies both the power rule and the chain rule in one go, violating the rule‑granularity requirement. Each step must change only one thing using a single named rule.
  • deepseek-r1:70b: pass
  • qwen3.6:27b-mlx: pass
Every verdict on record (15)
  • qwen3.6:27b-mlx: pass 2026-09-20
  • gpt-oss:20b: fail (error) 2026-09-20 — Step 4 applies both the power rule and the chain rule in one go, violating the rule‑granularity requirement. Each step must change only one thing using a single named rule.
  • qwen3.6:27b-mlx: pass 2026-09-20
  • gpt-oss:20b: pass 2026-09-20
  • 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 2026-09-19
  • 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
  • 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.