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.

Which lines are parallel? justify your answer. lines p and q are parallel because same side interior angles are congruent. lines p and q are parallel because alternate exterior angles are congruent lines l and m are parallel because same side interior angles are supplementary lines l and m are parallel because alternate interior angles are supplementary.