Sec(a-b) = cos(a+b) / cos^2a - sin^2b I'm trying to prove this, by turning the left side into the right. I am stuck. Presumably you have already done the sum and difference formulas for sin and cos.

Trigonometric sums and summation formulas have long served as essential tools for elucidating periodic phenomena across mathematics and the physical sciences. These techniques entail the explicit ...