#cos theta = csc theta -: cot theta#
#cos theta= 1/sin theta -: cos theta/sin theta#
# cos theta = 1/cancel(sin theta) * cancel(sin theta)/cos theta#
# cos^2 theta = 1#
# cos theta = +-1#
#theta = 0^@, 180^@, 360^@,...# or
#theta = npi#, #n = 0, 1, 2, 3, ...#