PDA

View Full Version : Indefinate integral for trig function


interinfinity
Feb 21, 2008, 06:50 PM
I would like to know how to set up this specific integral...
I don't know how to do an integral symbol on here...

I need the integral of : cos^2(x)tan^5(x)
that's... cosine squared x times tangent fifth x

I have tried doing it by integration by parts, but that won't work because the integral of either part is tricky to get without a graphing calculator.

I'm trying to use trig identities, the only thing I can come up with is breaking tan^5(x) up into secent... OK I admit it I don't have a clue what I'm talking about. I hate trig. If no one responds to this post I won't shed a tear, I hate trig.

galactus
Feb 22, 2008, 04:53 AM
Wow, finally a problem with a little meat to it. Something besides the usual "Help me solve x=4 for x".

A lot of times these things take some insight to see the substitution. Besides, trig is cool.

I was a surveyor for years and used it everyday. I didn't have to integrate trig functions, but I used it.

\int{cos^{2}(x)tan^{5}(x)}dx

Make the sub \frac{sin^{5}(x)}{cos^{5}(x)}=tan^{5}(x)

Then you get \int\frac{sin^{5}(x)}{cos^{3}(x)}dx

\int\frac{sin(x)sin^{4}(x)}{cos^{3}(x)}dx

Now, if you let u=cos(x), \;\ -du=sin(x)dx, \;\ x=arccos(u)

Note that sin^{4}(arccos(u))=(u^{2}-1)^{2}

It whittles down to something fairly easy to integrate.

-\int\frac{(u^{2}-1)^{2}}{u^{3}}du=-\int{u}du+2\int\frac{1}{u}-\int\frac{1}{u^{3}}du