trig - drwls, Friday, May 27, 2011 at 10:28pm
sec(90 -theta) = 1/cos(90 - theta)
= 1/(cos90 cos theta + sin 90 sin theta)
= 1/sin theta
= csc theta
You could get the same result by considering a right triangle. The sec of one acute angle is the csc of the other.