Youtube videos by Julie Harland are organized at http://YourMathGal.com. We verify a Trig identity by using the basic identities and the identity for the cosine of a sum or difference. We prove that (cos(x+y))/(cosx·cosy) = 1 - tanx·tany using a trig proof. Aligned to Common Core State Standard: HSF-TF.9 <br />For more great math videos visit: www.youtube.com/user/videosbyjulieharland/videos
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