solve 2cos^2(x) - cos(x) = 0 on the interval 0<=x < 180

we start  y factoring and solving for each equation:

cos(x) (2cos(x) - 1) = 0 

this means: 

cos(x) = 0 and cos(x) = 1/2

from the first equation we get:   x = 90

and from the second equation using the known trigonometric triangles we get

x = 60

therefore x = 60, 90 in the interval asked.