# Math Help - double or half angle formula to solve

1. ## double or half angle formula to solve

don't know how to use double or half angle formula to solve the equation:

sin2x+cosx=0

in the interval [0, 2pi) and the answers are pi/2, 7pi/6, 3pi/2, 11pi/6

2. Originally Posted by guardofthecolor4ever
don't know how to use double or half angle formula to solve the equation:

sin2x+cosx=0

in the interval [0, 2pi) and the answers are pi/2, 7pi/6, 3pi/2, 11pi/6
Substitute $\sin (2x) = 2 \sin x \cos x$.

Now factorise the resulting expression. Now apply the null factor law to get two simple trig equations that you need to solve over the given interval:

$\cos x = 0$

$2 \sin (x) + 1 = 0$

3. thanks!