Solve the equation on the interval 0 lessthanorequalto theta

given

cos( 2(theta) - (pi/2) ) = -1

( 2(theta) - (pi/2) ) = cos^-1 ( -1)

( 2(theta) - (pi/2) ) = cos^-1 ( cos (pi) )

( 2(theta) - (pi/2) ) = pi , 3pi

2 (theta) = pi + pi/2 , 2 (theta) = 3pi + pi/2

2 (theta) = 3pi/2 , 2 (theta) = 7pi/2

theta = 3pi/4, 7pi/4