Tuesday, 13 November 2018

trigonometry - How to prove this trigonometric identity of sine of n angles as sum?

How can we prove
sin(A1+A2+...+An)=cosA1.cos(A2)...cos(An)[S1S3+S5...]
where Sn denotes sum of tangents of angles taken n at a time.
I tried proving it but failed. I can derive it easily for n = 2 and 3 but not for general case. Wikipedia has same kind of formula for tangent but it is not derived.
https://en.m.wikipedia.org/wiki/List_of_trigonometric_identities
Please give a very simple detailed proof.

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