prove cos^2a +cos^2(a+120) cos^2(a-120)=3/2 - Maths -

Question

prove
cos^2a +cos^2(a+120) cos^2(a-120)=3/2

Tushar Kohli · Accepted Answer

Please check the expression written by you

The correct expression should be to prove: cos2a
+ cos2 (a + 120) + cos2 (a - 120) =
3/2

Solution:

cos2a + cos2 (a + 120) +
cos2 (a - 120) = 3/2

L H S

Using cos2A
= (1 + cos2A) / 2, we get