# Thread: Sum and difference formula

1. ## Sum and difference formula

Stuck on this question:

By using the sum and different formulas for cos(phi+theta) and cos(phi-theta) with phi=1/2(A+B) and theta=1/2(A-B),

(i) derive the formula
cos A+cos B=2 cos(1/2(A+B))cos(1/2(A-B))

(ii) and then must I derive the formula
cos A-cos B= -2 sin(1/2(A+B))sin(1/2(A-B)) from part (i) above.

Thanks!
Brigitte

2. It's ok. I have managed to solve this now. It was much easier than I had been thinking!

3. Excellent!
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