# Trig Proof 6

Taught by YourMathGal
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Meets NCTM Standards:
Lesson Summary:

In this lesson, Julie Harland teaches us how to verify a trig identity using basic identities and the identity for the cosine of a sum or difference. She takes us step-by-step through the process of proving that (cos(x+y))/(cos(x - y) = (coty - tanx)/(coty + tanx) by simplifying one side at a time and changing cotangents and tangents into sines and cosines. She also demonstrates how to multiply the numerator and denominator by the least common denominator to get rid of a complex fraction. By the end of the lesson, we learn how to do a formal proof or a more informal proof, both of which are valid.

Lesson Description:

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))/(cos(x - y) = (coty - tanx)/(coty + tanx) using a trig proof.

More free YouTube videos by Julie Harland are organized at http://yourmathgal.com

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