How to solve a trigonometric equation?

Question

Mathematics

Good day to all! I haven’t done trigonometry for a long time, now the time has come, there is such an equation, you need to simplify it (1-cosa) (1 + cosa), I’ve been struggling for 5 hours already. If possible, please explain the solution. Maybe someone knows good sites for studying mathematics, preparing for the exam, tell me please, I will be very grateful

Answer 1

(1 - cosX)(1 + cosX) = 1 - cos(x)^2 = sin(x)^2
Only it's not an equation. Where is the right side?

Answer 2

Use the Euler formulas. They turn trigonometry into rational exponent expressions that are easy to work with.