# Math Help - URGENT: Trig Identities Help

1. ## URGENT: Trig Identities Help

Merci!

2. Originally Posted by Lolcano
So, I need help with this problem.

Solve:

cos 2 x(2 cos x+1) = 0 for 0 <= x < 360
$\cos(2x)[2\cos{x} + 1] = 0$

set each factor equal to 0 ...

$\cos(2x) = 0$

$2x = 90 \, , \, 270 \, , \, 450 \, , \, 630$

$x = 45 \, , \, 135 \, , \, 225 \, , \, 315$

$\cos{x} = -\frac{1}{2}$

$x = 120 \, , \, 240$

3. Originally Posted by Lolcano
So, I need help with this problem.

Solve:

cos 2 x(2 cos x+1) = 0 for 0 <= x < 360

Using the double angle trig identities, cos 2 x can be simplified to:
2 cos^2 x - 1

2 cos^2 x - 1(2 cos x + 1)
Be careful, should be $(2 \cos^2 x - 1)(2 \cos x + 1)$

Expand this a make a substitution i.e. $\cos x = a$ and solve for $a$ using the quadratic formula