Given :
An expression (cos 6m)(cos 2m) .
To Find :
We need to express it in terms as sum or difference.
Solution :
We know,
cos( A + B ) = cosA cos B - sin A sin B
cos( A - B ) = cosA cos B + sin A sin B
Adding both the equations we get :
2cos A cos B = cos( A + B) + cos( A - B )
or
cos A cos B = cos( A + B) + cos( A - B )/2
Putting value of A = 6m and B = 2m in above equation, we get :
(cos 6m)(cos 2m) = cos( 6m + 2m ) + cos( 6m - 2m )/2
(cos 6m)(cos 2m) = cos(8m) + cos(4m)/2
Hence, this is the required solution.
