1. ## Cosine addition formula bump in the road.

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.

2. Originally Posted by DannyMath
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.
$\displaystyle (a-b)^2 = (a-b)(a-b) = a^2 - ab - ab + b^2 = a^2 - 2ab + b^2$

3. Oh thank goodness. I knew it had something to do with double brackets and FOIL. Thanks so much