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

Problem 2.899 · medium

Differentiate \( \displaystyle f(x) = \frac{\sqrt{\left(2 x + 1\right)^{2} + 1}}{2} \).
  1. \[ \frac{d}{d x} \frac{\sqrt{\left(2 x + 1\right)^{2} + 1}}{2} \]
    Start with the derivative of the function.✓ Proved
  2. \[ = \frac{\frac{d}{d x} \sqrt{\left(2 x + 1\right)^{2} + 1}}{2} \]
    constant-multiple rewritePull out the constant factor 1/2. Rewrite the square root as a power.✓ Proved
  3. \[ = \frac{\frac{d}{d x} \left(\left(2 x + 1\right)^{2} + 1\right)}{4 \sqrt{\left(2 x + 1\right)^{2} + 1}} \]
    chainApply the chain rule.✓ Proved
  4. \[ = \frac{\frac{d}{d x} \left(2 x + 1\right)^{2}}{4 \sqrt{\left(2 x + 1\right)^{2} + 1}} \]
    sumDifferentiate the sum inside the parenthesis.✓ Proved
  5. \[ = \frac{\left(4 x + 2\right) \frac{d}{d x} \left(2 x + 1\right)}{4 \sqrt{\left(2 x + 1\right)^{2} + 1}} \]
    powerApply the power rule to the squared term.✓ Proved
  6. \[ = \frac{8 x + 4}{4 \sqrt{\left(2 x + 1\right)^{2} + 1}} \]
    derivative algebraDifferentiate the linear term. Multiply the constants.✓ Proved
  7. \[ = \frac{2 x + 1}{\sqrt{\left(2 x + 1\right)^{2} + 1}} \]
    simplifySimplify the final expression.✓ Proved
Answer \( \frac{2 x + 1}{\sqrt{\left(2 x + 1\right)^{2} + 1}} \)

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 (2*x + 1)**2 + 1 = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where (2*x + 1)**2 + 1 = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where (2*x + 1)**2 + 1 = 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where (2*x + 1)**2 + 1 = 0
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where (2*x + 1)**2 + 1 = 0
9✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where (2*x + 1)**2 + 1 = 0
answer✓ Provedsympy 1.14.0final line against the stated answer: simplify(a - b) reduced to 0
undefined where (2*x + 1)**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 5 drops the derivative of the constant term +1. The sum rule should produce Derivative((2*x+1)**2, x)+Derivative(1, x), but the latter is zero and was omitted, violating the one‑rule‑per‑step rule.
  • qwen3.6:27b-mlx: pass
Every verdict on record (4)
  • qwen3.6:27b-mlx: pass 2026-09-26
  • gpt-oss:20b: fail (error) 2026-09-26 — Step 5 drops the derivative of the constant term +1. The sum rule should produce Derivative((2*x+1)**2, x)+Derivative(1, x), but the latter is zero and was omitted, violating the one‑rule‑per‑step rule.
  • qwen3.6:27b-mlx: pass 2026-09-26
  • gpt-oss:20b: pass 2026-09-26

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.