int_0^pi(xsin^4x)/(sin^4x+cos^4x)dxπ0xsin4xsin4x+cos4xdx = ??

1 Answer
May 3, 2018

pi^2/4π24.

Explanation:

We will use these Results :

(R_1) : int_0^af(x)dx=int_0^af(a-x)dx(R1):a0f(x)dx=a0f(ax)dx.

(R_2)(i) : int_0^(2a)f(x)dx=0, if f(2a-x)=-f(x), and,

(R_2)(ii) : int_0^(2a)f(x)dx=2int_0^af(x)dx, if f(2a-x)=f(x).

Let, I=int_0^pi (xsin^4x)/(sin^4x+cos^4x)dx.

:. I=int_0^pi{(pi-x)sin^4(pi-x)}/{sin^4(pi-x)+cos^4(pi-x)}dx,...[(R_1)]

=int_0^pi {(pi-x)sin^4x}/(sin^4x+cos^4x)dx,

=piint_0^pi sin^4x/(sin^4x+cos^4x)dx-int_0^pi(xsin^4x)/(sin^4x+cos^4x)dx,

i.e., I=piint_0^pi sin^4x/(sin^4x+cos^4x)dx-I,

or, 2I=piint_0^pi sin^4x/(sin^4x+cos^4x)dx.

Now, before applying (R_2), let us work out

f(2a-x)=f(pi-x)" for "f(x)=sin^4x/(sin^4x+cos^4x).

From above, we see that, f(pi-x)=f(x).

Hence, by (R_2)(ii),

(2I)/pi=2int_0^(pi/2)sin^4x/(sin^4x+cos^4x)dx,

or, I/pi=int_0^(pi/2)sin^4x/(sin^4x+cos^4x)dx...(1).

But for f(x)=sin^4x/(sin^4x+cos^4x),

f(pi/2-x)=sin^4(pi/2-x)/{sin^4(pi/2-x)+cos^4(pi/2-z)}dx,

=cos^4x/(cos^4x+sin^4x).

Therefore, by applying (R_1), on I/pi,

I/pi=int_0^(pi/2)cos^4x/(cos^4x+sin^4x)dx......(2).

Finally, we add (1) and (2) to get,

(2I)/pi=int_0^(pi/2){(sin^4x+cos^4x)}/{(cos^4x+sin^4x)}dx,

=int_0^(pi/2)1dx,

=[x]_0^(pi/2),

:. (2I)/pi=pi/2," giving, "

I=pi^2/4.

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