Definite Integrals!

Calculus Level 4

If y = f ( x ) y = f(x) makes positive intercept of 2 and 0 unit on x x and y y axis respectively and enclose an area of 3 4 \dfrac34 square units with the x x -axis, find 0 2 x f ( x ) d x \displaystyle \int_0^2 x f'(x) \, dx .

It is given that f ( x ) f(x) lies above the x x -axis in the interval ( 0 , 2 ) (0,2) .


This question is a part of the set Calculus .
3 4 \frac34 3 2 -\frac32 3 4 -\frac34 3 2 \frac32

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1 solution

Sahil Bansal
Mar 19, 2016

As per the question \text{As per the question } 0 2 f ( x ) d x = 3 4 \displaystyle \int_0^2 f(x)dx=\cfrac { 3 }{ 4 }

We can also evaluate the above integral using integration by parts: \text{We can also evaluate the above integral using integration by parts:}

0 2 1. f ( x ) d x = f ( x ) . 0 2 1 d x 0 2 f ( x ) . x d x \displaystyle \int_0^2 1.f(x)dx=f(x). \displaystyle \int_0^2 1dx- \displaystyle \int_0^2 f'(x).xdx

Hence \text{Hence } 0 2 x f ( x ) d x = f ( x ) . 0 2 1 d x 0 2 f ( x ) d x \displaystyle \int_0^2 xf'(x)dx=f(x). \displaystyle \int_0^2 1dx-\displaystyle \int_0^2 f(x)dx

= [ x f ( x ) ] 0 2 3 / 4 =[xf(x)]_0^2-3/4

= [ 2 f ( 2 ) 0 f ( 0 ) ] 3 / 4 = [2f(2)-0f(0)]-3/4

= [ 2 0 ] 3 / 4 = [2*0]-3/4
Since f(2)=0 \text{{Since f(2)=0}}

= 3 / 4 = -3/4

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