This is a question that Thelema asked me:

I don't have time to help at the moment so I'm asking if someone else can. Thanks!cos(s+t)cos(s-t)=cos^2s +cos^2t-1

so far i am at

[cos(s)cos(t)-sin(s)sin(t)][cos(s)cos(t)+sin(s)sin(t)] = cos^2(s)+cos^2(t)-1

I can multiply the left side but then i dont know

how to get it to equal the right. Can u plz help?

-Dan