Question 865061
cos (2θ) = cos^2 θ − (1/2)
-------
2cos^2(t)-1 = cos^2(t) - (1/2)
cos^2(t) = 1/2
-----------------
cos(t) = +-sqrt(2)/2)
t = pi/4 or (3/4)pi or (5/4)pi or (7/4)pi
---------------------------------------------
Cheers,
Stan H.