Question

If theeta =30°, verify that cos3theeta=4cos³theeta-3cos theeta.

Answer 1

Solution :

Here I am using A instead of theta.

A = 30°

LHS = cos 3A

= cos ( 3 × 30 )

= cos 90°

= 0 ----( 1 )

RHS = 4cos³A - 3cosA

= 4 cos³ 30° - 3 cos 30°

= 4 × (√3/2 )³ - 3 × ( √3/2 )

= 4 × ( 3√3 )/8 - 3√3/2

= 3√3/2 - 3√3/2

= 0 ---( 2 )

Therefore ,

LHS = RHS

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