Question

angle A=30°, then show that cos 3A = 4 COS^3^A - 3 COS A​

Answer 1

Step-by-step explanation:

Given that ∠A = 30°

LHS ---- 3A = 3 × 30 = 90°

            cos 90° = 0

RHS ----- 4cos³A - 3cosA

             = 4 cos³ 30° - 3 cos30°

             = 4 [√3 /2]³ - 3 [√3 /2]

             = 4 × 3√3 /8 - 3√3 /2

             = 3√3 /2 - 3√3 /2

             = 0

      LHS = RHS Hence Proved