Question

The expression cos(A-B) - cos(A+B) is equal to

1) -2sinAsinB
2) -2cosB
3) 2cosAcosB
4) 2sinAsinB

So far, all I've gotten is

cosxcosy + sinxsiny - cosxcosy - sinxsiny

I don't know where to go from there. I'm assuming that's zero, and I don't know which of the others equal 0... they all look the same.

Answer 1

expand them
cos a+b =cos a cos b - sin a sin b
cos a-b =cos a cos b+ sin a sin b
then subtract of them is
2sin a sinb