Wednesday, November 1, 2017

trigonometry - Product of projections of equispaced rotating vector



When equal and equi-spaced forces are summed on y-axis what is vector sum? How do we derive the formula





n1k=1sinπkn=cotπ2n




( Formula given by Marco Cantarini in comments below. )



By a similar token, can





n1k=1sinπkn=2n2n




represent some physics force multiplication situation or any generalized law in which



this analogue is valid? (Formula mentioned by Jack D'Aurizio in a recent thread



Geometric proof of sin60sin40...).


Answer



With some preliminary manipulations, both the identities can be derived by regarding




ζk=sinπkn
as roots of a suitable Chebyshev polynomial, then applying Vieta's formulas - relations between the roots and the coefficients of a monic polynomial.


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