Answer:
Take RHS
Explanation:
After that pit the formula of Cos C + Cos D and cut the 2 as they will be canceled and you will get Cos (A+B) *Cos (A-B) so after that use their formula and it will come im form of (A+B) *(A-B) so put them as using the formula of Asquare - B square and hence proved . Ask me of u get problem. Any where ok