Question

sin 42 sin 48-cos 42 cos 48

Answer 1

We know sin(90-ý) = cos ý
So
Cos42cos48-cos42cos48=0

Answer 2

sin 42.sin48 - cos 42.cos48
=-(cos 42.cos48 - sin 42.sin48)
=-cos(42+48)
    since cos(A+B)=cos A.cos B - sin A.sin B
=-cos 90
=0