Question:medium

If \[ f(x)=\frac1{x^2}\int_{3}^{x}\left(2t-3f'(t)\right)\,dt, \] then \[ f'(3)= \]

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When an integral equation contains \[ \int_a^x F(t)\,dt, \] first multiply away any outside factor and then differentiate using the Fundamental Theorem of Calculus: \[ \frac{d}{dx}\int_a^x F(t)\,dt = F(x). \]
Updated On: Jul 9, 2026
  • \(-\dfrac12\)
  • \(\dfrac12\)
  • \(-\dfrac13\)
  • \(\dfrac13\) 

Show Solution

The Correct Option is B

Solution and Explanation

Concept: Use the Fundamental Theorem of Calculus: differentiate after multiplying by \(x^2\) to remove the integral sign, then use the original equation to find \(f(3)\).

Step 1:
\(f(x) = \frac{1}{x^2}\int_3^x (2t-3f'(t))dt \Rightarrow x^2 f(x) = \int_3^x (2t-3f'(t))dt\).

Step 2:
Differentiate: \(2x f(x) + x^2 f'(x) = 2x - 3f'(x)\).

Step 3:
From original, \(f(3) = \frac{1}{9}\int_3^3 ... dt = 0\).

Step 4:
Put \(x=3\) into differentiated equation: \(2(3)(0) + 9f'(3) = 6 - 3f'(3) \Rightarrow 12f'(3)=6 \Rightarrow f'(3)=1/2\).

Step 5:
Write the final answer. \(\boxed{\frac12}\)
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