Question

Prove That:
cos( 120 + a ) cos( 120 - a ) = (2 cos2a - 1 )/4

Answer 1

Hi friend!!

We know that cos(A+B)cos(A-B)=cos²A-sin²B

Now, cos( 120 + a ) cos( 120 - a )

=cos²120-sin²a

=(1/2)²-sin²a

=(1/4)-(1-cos2a)/2

=(1-2+2cos2a)/4

=(2cos2a-1)/4

Hence proved!

I hope this will help you;)