# Math Help - double angle identity

1. ## double angle identity

4sin2Xcos2X
A) 1/2sin16X
B) cos8X
C) cos4X
D)2sin4X

determine whether the equation is an identity. if an identity prove it.

(sinx+cosx)^2=1

find all solutions in [0,2pi)
tanxsecx=squareroot of 2.

2sinxcosx+cosx=0

thank you so much in advanced!!!!!

2. Originally Posted by amanda0603
4sin2Xcos2X
A) 1/2sin16X
B) cos8X
C) cos4X
D)2sin4X
use $\sin 2x = 2\sin x \cos x$

determine whether the equation is an identity. if an identity prove it.

(sinx+cosx)^2=1
Expand this and compare with $\sin^2 x + \cos^2 x = 1$, what can you deduce?

find all solutions in [0,2pi)
tanxsecx=squareroot of 2.
Try using $\tan x = \frac{\sin x}{\cos x}$ and $\sec x = \frac{1}{\cos x}$

2sinxcosx+cosx=0

thank you so much in advanced!!!!!
Pull out a $\cos x$ and solve each part individually (set them equal to zero)

Good luck