If A, B ,C are the angles of a triangle then,
Please help me to prove this!

 see proof below

Step-by-step explanation:

Given: A + B + C = π                     → A + B = π - C

                                                     → C = π - (A + B)

Use the Cofunction Identities:      sin (π/2 - A) = cos A

                                                       cos (π/2 - A) = sin A

Use the Double Angle Identity: cos 2A = 1 - 2 sin² A

Use Sum to Product Identity:  cos A - cos B = 2 sin [(A+B)/2] · sin [(A-B)/2]

Proof LHS → RHS:

LHS = RHS