Question

if cos(theta) = 1
what is theta

Answer 1

recall (x.y) on circle = (cos(t),sin(t))
so from the trig circle, x = 1 when t=0, 2pi , 4pi ,...

Answer 2

cos x is 1 when x = 0 and 2pi,

o pi , pi/2 , pi , 3/4pi, 2 pi

or
0 degree , 90 , 180 , 270, 360

and cos starts at 1 at 0 degrees, then 0 at 90, -1 at 180, 0 at 270, and back at 1 at 360