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)
www.ies.co.jp

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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