Sine and cosine at max are 1. Cosine is X and Sine is Y. The values of them repeat as their graphs are waves. As you can see here
So here `pi/4` and `7pi/4` can both equal theta
theta can be 45 degrees or it can be 315 degrees
`cos(theta)` is positive in the 1st and 4th quadrants.
`cos(theta) = -sqrt(2)/2 = 1/sqrt(2)`
=> `theta = sin^-1(1/sqrt(2))`
`theta = 45 ^@ or 315^@ ` in degrees
`theta = pi/4 or (7pi)/4` in radians
`cos(theta)` is positive in the 1st and 4th quadrants. Looking at a unit circle, the values of theta that satisfy these conditions are 45 and 315 degrees.