Cosine and Sine of the Sum of Two Angles
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The formulas for 
and 
are derived using purely geometric reasoning, avoiding the usual cumbersome algebraic manipulations. Proceed through steps 1–5 to follow the derivation.
Contributed by: Tomas Garza (July 2011)
Open content licensed under CC BY-NC-SA
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"Cosine and Sine of the Sum of Two Angles"
http://demonstrations.wolfram.com/CosineAndSineOfTheSumOfTwoAngles/
Wolfram Demonstrations Project
Published: July 1 2011
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