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 they all look the same.

Cos (A-B) - cos (A+B) 
= (cosAcosB +sinAsinB) - (cosAcosB - sinAsinB)
=2sinAsinB

Ans: 4

Cos (A+B)=cos (A) cos( B)-sin (A) sin (B)
cos (A-B)=cos (A) cos (B)+sin (A)sin (B)

Then:
cos (A-B)-cos (A+B)=
[cos (A) cos (B)+sin (A)sin (B)]-[cos (A) cos( B)-sin (A) sin (B)]=
cos (A) cos( B)+sin (A) sin (B)-cos (A) cos( B)+sin (A) sin (B)=
sin (A)sin (B)+Sin (A)sin(B)=
2sin (A)sin (B)

4) 2sin (A)sin (B)