Hi guys,

I'm reading up on the proof for the addition formula for cosine in my precal book. But I'm actually having a bit of trouble with a more elementary issue. It's simply what the steps are to expanding:

(cos t - cos s)^2

to

cos^2 t - 2cos s cos t + cos^2 s

I've been looking for the answer but unfortunately my elementary algebra textbook is back at home and I'm at work now. Any enlightenment is appreciated.