# TI-89 Can't integrate trig function. Why?

• Sep 5th 2010, 06:18 PM
eay444
TI-89 Can't integrate trig function. Why?
I'm trying to use my TI-89 Titanium calculator to integrate the function sin(2x)/(1+sin x) with respect to x and when I type it in and press enter it gives me the same exact expression I just typed in. I'm pretty sure I'm typing the format in right because i've successfully integrated many other integrals. Does it have something to do with the fact that the function can only be integrated using trig identites? Does anybody know what's going on?
• Sep 5th 2010, 07:18 PM
skeeter
sometimes, if an elementary closed-form antiderivative for the integral exists, a clever substitution is required that the CAS of the calculator is incapable of figuring out.

it will also occur when the integral has no elementary closed-form antiderivative.
• Sep 6th 2010, 03:10 AM
mr fantastic
Quote:

Originally Posted by eay444
I'm trying to use my TI-89 Titanium calculator to integrate the function sin(2x)/(1+sin x) with respect to x and when I type it in and press enter it gives me the same exact expression I just typed in. I'm pretty sure I'm typing the format in right because i've successfully integrated many other integrals. Does it have something to do with the fact that the function can only be integrated using trig identites? Does anybody know what's going on?

If you write sin(2x) = 2 sin(x) cos(x) there is an obvious substitution that leads to a simple integral. There is a moral to the story.