Question

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Answer 1

cos 3a = 4 cos ^3a-- 3cos a    given,,,  a = 30 degree

cos 90 = 4cos^3 30 -- 3cos 30

0 = 4 × [√3/2]^3  -- 3 × √3/2

r.h.s  

3√3/2 -- 3√3/2  

0  

lhs = rhs

proved

Answer 2

I hope it will help you.....