Posted by **Juliet** on Sunday, September 18, 2011 at 2:16pm.

differentiate:

y=sin^2(x)- cos^2(x)

I have this:

y'= 2cosx + 2sinx

What do i do next??

- Calculus -
**Reiny**, Sunday, September 18, 2011 at 5:01pm
Ahhh, you might recognize that

cos^2 x - sin^2 x = cos 2x

so y=sin^2(x)- cos^2(x)

= - cos 2x

dy/dx = 2sin 2x or 4sinxcosx

If you don't see that identity right away, then

dy/dx = 2(sinx)cosx - 2cosx(-sinx)

= 4sinxcosx or 2sin(2x)

- Calculus -
**Anonymous**, Monday, September 19, 2011 at 10:39am
Oh! Thank you so much! I completely forget about the double angle identity!!! :)

THANKS!!!

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