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

Problem 2.244 · medium

Differentiate \( \displaystyle f(x) = \sqrt{\left(2 x - 1\right)^{2} + 1} \).
  1. \[ \frac{d}{d x} \sqrt{\left(2 x - 1\right)^{2} + 1} \]
    derivativeStart with the derivative of the function.✓ Proved
  2. \[ = \frac{\frac{d}{d x} \left(\left(2 x - 1\right)^{2} + 1\right)}{2 \sqrt{\left(2 x - 1\right)^{2} + 1}} \]
    chainApply the chain rule for the square root function.✓ Proved
  3. \[ = \frac{\frac{d}{d x} 1 + \frac{d}{d x} \left(2 x - 1\right)^{2}}{2 \sqrt{\left(2 x - 1\right)^{2} + 1}} \]
    sumApply the sum rule to the inner expression.✓ Proved
  4. \[ = \frac{\left(4 x - 2\right) \frac{d}{d x} \left(2 x - 1\right)}{2 \sqrt{\left(2 x - 1\right)^{2} + 1}} \]
    powerApply the power rule and chain rule to the squared term.✓ Proved
  5. \[ = \frac{8 x - 4}{2 \sqrt{\left(2 x - 1\right)^{2} + 1}} \]
    derivative algebra algebraDifferentiate the linear term. Multiply the constants in the numerator. Combine the terms into a single fraction.✓ Proved
  6. \[ = \frac{4 x - 2}{\sqrt{\left(2 x - 1\right)^{2} + 1}} \]
    simplifySimplify the fraction by canceling the common factor of 2.✓ Proved
  7. \[ = \frac{4 x - 2}{\sqrt{4 x^{2} - 4 x + 2}} \]
    algebra simplifyExpand the squared binomial inside the square root. Combine the constant terms inside the square root.✓ Proved
  8. \[ = \frac{\sqrt{2} \left(4 x - 2\right)}{2 \sqrt{2 x^{2} - 2 x + 1}} \]
    algebraFactor out a 2 from the expression inside the square root.✓ Proved
  9. \[ = \frac{\sqrt{2} \left(2 x - 1\right)}{\sqrt{2 x^{2} - 2 x + 1}} \]
    algebraWait, let's simplify the coefficient instead.✓ Proved
  10. \[ = \frac{2 \left(2 x - 1\right)}{\sqrt{4 x^{2} - 4 x + 2}} \]
    algebraIncorrect logic in previous thought, let's use a clean simplification.✓ Proved
  11. \[ = \frac{\sqrt{2} \left(2 x - 1\right)}{\sqrt{2 x^{2} - 2 x + 1}} \]
    algebra algebraLet's re-evaluate the simplification of (2/sqrt(2)). Simplify the constant factor.✓ Proved
Answer \( \frac{4 x - 2}{\sqrt{\left(2 x - 1\right)^{2} + 1}} \)

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. A reviewer's objection to the explanation stands; see 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
undefined where (2*x - 1)**2 + 1 = 0
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where (2*x - 1)**2 + 1 = 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
undefined where 4*x**2 - 4*x + 2 = 0
10✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 4*x**2 - 4*x + 2 = 0
11✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 4*x**2 - 4*x + 2 = 0
undefined where 2*x**2 - 2*x + 1 = 0
12✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 2*x**2 - 2*x + 1 = 0
13✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 2*x**2 - 2*x + 1 = 0
undefined where 4*x**2 - 4*x + 2 = 0
14✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 4*x**2 - 4*x + 2 = 0
undefined where 2*x**2 - 2*x + 1 = 0
15✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 2*x**2 - 2*x + 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 4 applies both the power rule and the chain rule but labels only "power", violating the one‑rule‑per‑step rule. Additionally, the subsequent simplifications (steps 12–15) are incorrect and lead to a final expression that does not equal the correct derivative.
  • deepseek-r1:70b: fail (misleading) — Step 12 introduces incorrect logic that could mislead a student, though it's corrected later.
  • qwen3.6:27b-mlx: fail (error) — Step 4 applies both the power rule and the chain rule simultaneously, violating the one-rule-per-step constraint. Additionally, the solution performs unnecessary and confusing algebraic manipulations in steps 9-15 that deviate from the stated answer without justification.
Every verdict on record (12)
  • qwen3.6:27b-mlx: fail (error) 2026-09-20 — Step 4 applies both the power rule and the chain rule simultaneously, violating the one-rule-per-step constraint. Additionally, the solution performs unnecessary and confusing algebraic manipulations in steps 9-15 that deviate from the stated answer without justification.
  • gpt-oss:20b: fail (error) 2026-09-20 — Step 4 applies both the power rule and the chain rule but labels only "power", violating the one‑rule‑per‑step rule. Additionally, the subsequent simplifications (steps 12–15) are incorrect and lead to a final expression that does not equal the correct derivative.
  • qwen3.6:27b-mlx: fail (error) 2026-09-20 — Step 4 applies both the power rule and the chain rule simultaneously, violating the constraint that each step must change only one thing. Additionally, the final answer in step 15 is algebraically incorrect (it introduces a factor of sqrt(2) not present in the correct derivative).
  • gpt-oss:20b: fail (error) 2026-09-20 — Step 12 (and subsequent steps) incorrectly manipulate the fraction, leading to a final expression that is not algebraically equivalent to the correct derivative. The simplification from step 11 to step 12 applies a division by a fraction incorrectly, breaking the one‑rule‑per‑step rule and producing an incorrect final result.
  • qwen3.6:27b-mlx: fail (error) 2026-09-19 — Step 4 applies both the power rule and the chain rule simultaneously, violating the one-rule-per-step constraint. Additionally, the final answer in step 15 is algebraically incorrect; the simplification of the constant factor 2/sqrt(2) results in sqrt(2), but the numerator was already simplified from 4*(2x-1) to 2*(2x-1) in step 8, so the final result should be (2x-1)/sqrt(2x^2-2x+1), not sqrt(2)*(2x-1)/sqrt(2x^2-2x+1).
  • deepseek-r1:70b: fail (misleading) 2026-09-19 — Step 12 introduces incorrect logic that could mislead a student, though it's corrected later.
  • gpt-oss:20b: fail (error) 2026-09-19 — Step 4 applies both the power rule and the chain rule to differentiate (2*x-1)**2, yet it is labeled only as "power". This violates the rule that each step must change only one thing and be labeled with the correct rule.
  • qwen3.6:27b-mlx: fail (error) 2026-09-19 — The final answer is mathematically incorrect; the solver incorrectly simplified the constant factor 2/sqrt(2) to sqrt(2) while simultaneously dropping a factor of 2 from the denominator's radical, resulting in a value twice as large as the correct derivative. Specifically, step 15 claims 2/sqrt(2) = sqrt(2) (correct) but applies it to a denominator that was already simplified incorrectly in steps 9-14, leading to a final result of sqrt(2)*(2x-1)/sqrt(2x^2-2x+1) instead of the correct (2x-1)/sqrt(x^2-x+0.5) or equivalent. The stated answer (4x-2)/sqrt((2x-1)^2+1) is correct, but the derivation ends at a different, incorrect expression.
  • deepseek-r1:70b: fail (misleading) 2026-09-19 — Steps 11-15 introduce unnecessary complexity that could confuse a student.
  • gpt-oss:20b: fail (error) 2026-09-19 — Step 4 applies both the power rule and the chain rule in one line, violating the one‑rule‑per‑step rule. Step 5 combines the derivative of the linear term with an algebraic multiplication, again applying two rules at once. Subsequent steps 12–15 contain incorrect algebraic manipulations (e.g., the transition from step 11 to 12 is invalid), leading to an incorrect final expression.
  • gpt-oss:20b: inconclusive 2026-09-17 — reviewer response could not be parsed:
  • 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.