integrate?

Forum for the GRE subject test in mathematics.
hadimotamedi
Posts: 34
Joined: Thu Dec 30, 2010 4:36 am

integrate?

Postby hadimotamedi » Sat Jun 30, 2012 6:19 am

Dear All
I need to compute the following integral :
f(x)=(1+sin(x))/((cos(x))^2)
I=integral(f(x)dx)
Can you please propose a simple change of variable to simplify its calculation?
Thank you

vonLipwig
Posts: 51
Joined: Sat Mar 17, 2012 9:58 am

Re: integrate?

Postby vonLipwig » Sat Jun 30, 2012 6:40 am

You don't need to change variables, just break it up into two fractions (one for each term in the numerator).

Then it comes down to integrating 1/(cos^2) and sin/(cos^2), or, if you prefer, sec^2 and sec*tan. Those are integrals you're probably familiar with.

hadimotamedi
Posts: 34
Joined: Thu Dec 30, 2010 4:36 am

Re: integrate?

Postby hadimotamedi » Sat Jun 30, 2012 7:05 am

Thank you very much for your help. So the integral breaks into (sec(x))^2 + sec(x).tan(x)
For the fist term, it is straightforward but what about the second term?

vonLipwig
Posts: 51
Joined: Sat Mar 17, 2012 9:58 am

Re: integrate?

Postby vonLipwig » Sat Jun 30, 2012 7:32 am

The derivative of sec(x) is sec(x)tan(x).

hadimotamedi
Posts: 34
Joined: Thu Dec 30, 2010 4:36 am

Re: integrate?

Postby hadimotamedi » Sat Jun 30, 2012 7:56 am

Thank you very much for your reply. I got the point clearly.

vonLipwig
Posts: 51
Joined: Sat Mar 17, 2012 9:58 am

Re: integrate?

Postby vonLipwig » Sat Jun 30, 2012 8:25 am

No worries =)

tcbh
Posts: 7
Joined: Tue May 15, 2012 5:11 pm

Re: integrate?

Postby tcbh » Sun Jul 01, 2012 7:47 pm

So do you just have to memorize these trig integrals?

vonLipwig
Posts: 51
Joined: Sat Mar 17, 2012 9:58 am

Re: integrate?

Postby vonLipwig » Sun Jul 01, 2012 8:38 pm

The ones we just used are pretty basic, so yes, I'd remember them, as well as the fact that those for cosec and cot look similar but with some minus signs. Anything more complicated, I'd substitute for sin, cos or tan and try and work it out that way, rather than trying to remember.




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