# Compute the integral using trig substitution

#### imattxc

##### New member
I need to correct the following problem using trig substitution.

I keep getting stuck at:

If I dont use trig sub and just use u-sub I get like 2/15, but I think I need to answer using trig sub. Thanks for any help you can offer!

#### Subhotosh Khan

##### Super Moderator
Staff member
I need to correct the following problem using trig substitution.
View attachment 4649

I keep getting stuck at:

View attachment 4648

If I dont use trig sub and just use u-sub I get like 2/15, but I think I need to answer using trig sub. Thanks for any help you can offer!

sin[SUP]3[/SUP](Θ) * cos[SUP]2[/SUP](Θ) = sin(Θ) * [1-cos[SUP]2[/SUP](Θ)] * cos[SUP]2[/SUP](Θ) = sin(Θ) * cos[SUP]2[/SUP](Θ) - sin(Θ) * cos[SUP]4[/SUP](Θ) .... Now integrate....

#### HallsofIvy

##### Elite Member
Typically, if you have an odd power of sine or cosine you can factor one out, to use with the "dx", then use $$\displaystyle sin^2(x)+ cos^2(x) = 1$$, so that either $$\displaystyle sin^2(x)= 1- cos^2(x)$$ or $$\displaystyle cos^2(x)= 1- sin^2(x)$$ to reduce the remaining even power. Here, you have $$\displaystyle \int_0^{\pi/2} sin^3(\theta) cos^2(\theta)d\theta$$ which we can write as $$\displaystyle \int_0^{\pi/2} sin^2(\theta) cos^2(\theta) (sin(\theta) d\theta)= \int_0^{\pi/2} (1- cos^2(\theta))cos^2(\theta)(sin(\theta)d\theta)= \int_0^{\pi/2} (cos^2(\theta)- cos^4(\theta))(sin(\theta)d\theta)$$.

Now make the substitution $$\displaystyle u= cos(\theta)$$.

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