Derivative of \( \displaystyle \frac{\sqrt{4 x^{2} + 1}}{2} \)
Problem 2.50 · medium
Differentiate \( \displaystyle f(x) = \frac{\sqrt{4 x^{2} + 1}}{2} \).
- \[ \frac{d}{d x} \frac{\sqrt{4 x^{2} + 1}}{2} \]derivativeStart with the derivative of the function.✓ Proved
- \[ = \frac{\frac{d}{d x} \sqrt{4 x^{2} + 1}}{2} \]constant rewritePull out the constant factor 1/2. Rewrite the square root as a fractional power.✓ Proved
- \[ = \frac{\frac{d}{d x} \left(4 x^{2} + 1\right)}{4 \sqrt{4 x^{2} + 1}} \]chain algebraApply the chain rule. Simplify the constant coefficient.✓ Proved
- \[ = \frac{\frac{d}{d x} 1 + \frac{d}{d x} 4 x^{2}}{4 \sqrt{4 x^{2} + 1}} \]sumApply the sum rule to the inner function.✓ Proved
- \[ = \frac{\frac{d}{d x} 4 x^{2}}{4 \sqrt{4 x^{2} + 1}} \]constantThe derivative of the constant 1 is 0.✓ Proved
- \[ = \frac{2 x}{\sqrt{4 x^{2} + 1}} \]power algebra simplifyApply the power rule to 4*x**2. Move the negative exponent to the denominator. Simplify the fraction by dividing 8 by 4.✓ Proved
Answer \( \frac{2 x}{\sqrt{4 x^{2} + 1}} \)
✓ 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
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | ✓ Proved | sympy 1.14.0 | line 1 is the problem as stated |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 4*x**2 + 1 = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 4*x**2 + 1 = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 4*x**2 + 1 = 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 4*x**2 + 1 = 0 |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 4*x**2 + 1 = 0 |
| 9 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 4*x**2 + 1 = 0 |
| 10 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 4*x**2 + 1 = 0 |
| answer | ✓ Proved | sympy 1.14.0 | final line against the stated answer: simplify(a - b) reduced to 0 undefined where 4*x**2 + 1 = 0 |
| answer, a second way | ✓ Proved | sympy 1.14.0 | SymPy differentiated f directly and got the stated answer |
Reviewers
gpt-oss:20b: passdeepseek-r1:70b: passqwen3.6:27b-mlx: pass
Every verdict on record (12)
qwen3.6:27b-mlx: pass 2026-09-20gpt-oss:20b: pass 2026-09-20qwen3.6:27b-mlx: fail (style) 2026-09-20 — Step 5 applies two rules at once: it simplifies the constant coefficient (1/2 * 1/2 = 1/4) and rewrites the inner function (4*x**2 to 4*x*x). Step 8 is also defective because it applies the constant-multiple rule (derivative of 4*x**2 is 4*2*x) and the power rule simultaneously, rather than isolating the constant factor first.gpt-oss:20b: pass 2026-09-20qwen3.6:27b-mlx: fail (style) 2026-09-19 — Step 5 applies two rules at once: it simplifies the coefficient (1/2 * 1/2 = 1/4) and rewrites the inner function (4*x**2 to 4*x*x). Step 8 is labeled 'power' but effectively applies the constant-multiple rule to 4*x**2; while 'power' is acceptable for the core operation, the label is imprecise given the constant factor, but the primary defect is the multi-rule step 5.deepseek-r1:70b: pass 2026-09-19gpt-oss:20b: pass 2026-09-19qwen3.6:27b-mlx: pass 2026-09-19deepseek-r1:70b: fail (misleading) 2026-09-19 — Step 5 note incorrectly describes simplifying a constant coefficient.gpt-oss:20b: pass 2026-09-19gpt-oss:20b: pass 2026-09-17deepseek-r1:70b: fail 2026-09-17 — Step 5's note is misleading; it incorrectly describes simplifying a constant coefficient instead of expanding the term for differentiation.
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.