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JEE Main 2020
Differentiation
Differentiation
Hard

Question

For the differentiable function f:R{0}Rf: \mathbb{R}-\{0\} \rightarrow \mathbb{R}, let 3f(x)+2f(1x)=1x103 f(x)+2 f\left(\frac{1}{x}\right)=\frac{1}{x}-10, then f(3)+f(14)\left|f(3)+f^{\prime}\left(\frac{1}{4}\right)\right| is equal to

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Solution

Given the equation: 3f(x)+2f(1x)=1x103f(x) + 2f\left(\frac{1}{x}\right) = \frac{1}{x} - 10 Replace xx with 1x\frac{1}{x} in the original equation: 3f(1x)+2f(x)=x103f\left(\frac{1}{x}\right) + 2f(x) = x - 10 Now, we have two equations: 3f(x)+2f(1x)=1x103f(x) + 2f\left(\frac{1}{x}\right) = \frac{1}{x} - 10 3f(1x)+2f(x)=x103f\left(\frac{1}{x}\right) + 2f(x) = x - 10 By adding the two equations, we can find f(x)f(x): 5f(x)=3x2x105f(x) = \frac{3}{x} - 2x - 10 Now, let's differentiate both sides with respect to xx: 5f(x)=3x225f'(x) = -\frac{3}{x^2} - 2 Now, we can find the values for f(3)f(3) and f(14)f'\left(\frac{1}{4}\right): f(3)=15(1610)=3f(3) = \frac{1}{5}(1 - 6 - 10) = -3 f(14)=15(482)=10f'\left(\frac{1}{4}\right) = \frac{1}{5}(-48 - 2) = -10 Finally, calculate the expression we are interested in : f(3)+f(14)=310=13\left|f(3) + f'\left(\frac{1}{4}\right)\right| = \left|-3 - 10\right| = 13

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