Sum of Trigonometric Functions Orthogonal to Each Other
📂Fourier AnalysisSum of Trigonometric Functions Orthogonal to Each Other
Define Cn and Sn as follows.
Cn:Sn:=1+cosx+cos2x+⋯+cosnx=sinx+sin2x+⋯+sinnx
Then, the following equation holds.
CnSn=sin21xsin2n+1xcos2nx=sin21xsin2n+1xsin2nx
Proof
Use the Euler’s formula.
=========Cn+iSn (1+cosx+cos2x+⋯+cosnx)+i(sinx+sin2x+⋯+sinnx) 1+(cosx+isinx)+(cos2x+isin2x)+⋯+(cosnx+isinnx) ei0x+ei1x+ei2x+⋯+einx k=0∑n(eix)k eix−1ei(n+1)x−1 eix/2ei(n+1)x/2eix/2−e−ix/2ei(n+1)x/2−e−i(n+1)x/2 einx/2sin21xsin2n+1x sin21xsin2n+1x(cos2nx+isin2nx) sin21xsin2n+1xcos2nx+isin21xsin2n+1xsin2nx
The fifth equality uses the formula for the sum of a geometric series.
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