We mentioned in class that expressing periodic functions as complex exponentials can be mathemati cally convenient. The key equation we use in doing so is: ti = cos 0 + isin 0 Use this expression to prove the following identities: (a) cos(wit) cos(w2t) = 2 {cos[(w1 + w2)t] + cos[(w1 - W2)t]} (b) cos( wt + 41) + cos(wt + 42) = | A| cos( wt + $) |A| = V2 + 2 cos(41 - 42 = arctan sin 41 + sin 42 cos 41 + cos 42