Express cos6mcos2m as a sum or difference

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.

The answer is 4/9

Hope this helps