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.

There are some trigonometric identities which you must remember in order to simplify trigonometric expressions when required. When you are asked to prove something, ignore the right hand side of the ...