Half Angle Formulas

Definition: Half angle formulas are identities which can be used to find exact values in trigonometric expressions:

Example: Given cos x = -15/17 and 180 <x < 270, find tan (x/2).


Step 1: Looking at the formula for tan (x/2), we can see cos x will be needed to solve. Plug cos x = -15/17 into the formula. Because x is in Quadrant III where tan is positive, the plug or minus sign becomes positive.Step 2: Simplify.


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