Function and Calculus.

Calculus Level 5

Let f ( x ) = 2 + 2 x ( t t f ( t ) ) d t f(x) = 2 + \int_{2}^{x}(t - tf(t))dt . Then find the value of f ( 0 ) f(0) correct upto three places of decimal.


The answer is 8.389.

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1 solution

f ( x ) = 2 + 2 x ( t t f ( t ) ) d t f(x) = 2 + \int_{2}^{x}(t - tf(t))dt

Differentiating both sides w.r.t x x , we get

f ( x ) = x x f ( x ) f'(x) = x - xf(x)

This is a linear differential equation. Its solution is ( e ( x 2 2 ) ) f ( x ) = x e ( x 2 2 ) + c (e^{(\frac{x^{2}}{2})})f(x) =\int xe^{(\frac{x^{2}}{2})} + c

\implies ( e ( x 2 2 ) ) f ( x ) = e ( x 2 2 ) + c (e^{(\frac{x^{2}}{2})})f(x) =e^{(\frac{x^{2}}{2})} + c

Since f ( 2 ) = 2 f(2) = 2 , so c = e 2 c = e^2 .

So, f ( x ) = 1 + ( e 2 ) ( e ( x 2 2 ) ) f(x) = 1 + (e^2)(e^{-(\frac{x^{2}}{2})})

Hence f ( 0 ) = 1 + e 2 = 8.389 f(0) = 1 + e^2 = 8.389

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