Question

evaluate cos 48° cos 42° - sin 48° sin 42°

Answer 1

It is a formula of cos(A+B)
Cos(48+42)
Then, cos 90
= 0.

Answer 2

We have to evaluate 
cos 48° cos 42° - sin 48° sin 42°
We know that, 
cos (A+B) = cos A cos B - sin A sin B
Here, A = 48° and B = 42°
Hence, with the help of the formula given above, we can write
cos 48° cos 42° - sin 48° sin 42° = cos (48° + 42°)
cos 48° cos 42° - sin 48° sin 42° = cos 90°
cos 48° cos 42° - sin 48° sin 42° = 0     (∵ cos 90°=0)
This is the required answer.
Hopefully it helps. Thank u.