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