Math / Trigonometry / Proof: cos(a+b) = (cos a)(cos b)-(sin a)(sin b)
An elementary proof of two formulas in trigonometry
www.math.wisc.edu
sin(A+B)
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Sum and Difference Formulas (Trig without Tears Part 7)
oakroadsystems.com
Sine, Cosine, and Ptolemy's Theorem
www.cut-the-knot.org
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