Is it possible to give a formal proof that the definition of cosinus by a series is the same as done in geometry using right triangles? If the answer is yes, then please give the proof here.
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Probably. Otherwise they most likely wouldn't have the same name. – Arthur May 26 '21 at 20:05
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The meat of such a proof would just be proving the derivatives of sine and cosine, from there it would just be a matter of using Taylor's theorem. – Stephen Donovan May 26 '21 at 20:09
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Yes, thank you! – Sven Wirsing May 26 '21 at 20:18