int_0^(pi/2) dx/(5+4cosx)?

1 Answer
Jun 13, 2017

2/3arc tan(1/3).

Explanation:

Let, I=int_0^(pi/2) dx/(5+4cosx).

The Proper Substn. for this type of Integrals is, tan(x/2)=t, so

that,

1/2sec^2(x/2)dx=dt, or, dx=(2dt)/sec^2(x/2)=(2dt)/(1+tan^2(x/2)),

i.e., dx=(2dt)/(1+t^2).

When, x=0, t=tan0=0, &, x=pi/2, t=tan(pi/4)=1.

Further, 5+4cosx=5+4{(1-tan^2(x/2))/(1+tan^2(x/2))}=5+4((1-t^2)/(1+t^2))

=(9+t^2)/(1+t^2).

:. I=int_0^1 {(2dt)/(1+t^2)}/{(9+t^2)/(1+t^2)}=2int_0^1dt/(t^2+3^2),

=2*1/3[arc tan (t/3)]_0^1,

=2/3[arc tan(1/3)-0],

:. I=2/3arc tan(1/3).

Enjoy Maths.!