Question

If a =30 then show that cos3a=4cos cube a -3cos
a

Answer 1

a = 30 °
cos a = cos 30° = √3 /2

RHS

=4 cos^3 a - 3 cos a

= cos a [4cos ^ 2 a - 3]

= √3/2 [4x3/4 - 3]

= √3/2 [0]

= 0

= cos 90°

= cos (3a)

= LHS

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