Use Identities to find the exact value cos 36 degree cos 24

Use Identities to find the exact value. cos 36 degree cos 24 degree - sin 36 degree sin 24 degree squareroot 2/2 1 1/2 squareroot 3/2

Solution

cos(A + B) = cos A cos B sin A sin B
in our case A=36 degrees
and B=24 degrees
so we an write cos36°cos24°-sin36°sin24° as cos(36+24)°=cos60°=1/2 is our answer
hence part C is correct.