Find the Exact Value of Cos(2B) Using a Double Angle Identity & Information About Angle B

Given tan(B)=15/8 and angle B is in quadrant III, the angle is drawn in standard position. As tan(B)=y/x=15/8, the values of x and y are found remembering that both x and y values are negative in quadrant III. The Pythagorean theorem is used to find r. Then the value of cos(2B) is found using one version of the double angle identity for the cosine of 2B.

Timestamps
0:00 Introduction
1:38 Draw Angle B in standard position
2:10 Find x, y, and r
4:00 Find cos(2B)

[rinaalexandracastillo]

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