■三角関数の無限乗積公式(その2)
(その1)を一般化すると,・・・
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sinx=sinx/2n・Σ(−1)^r/r!Π(2n−k)(2cosx/2n)^2n-2r-2
=2nsinx/2n・{Σ(−1)^r/r!Π(2n−k)(2cosx/2n}^2n-2r-2}/2n
したがって,
sinx/x=Π{Σ(−1)^r/r!Π(2n−k)(2cosx/2n}^2n-2r-2}/2n
=Π{Σ(−1)^r(2n,2r+1){sinx/(2n)^k}^2r{cosx/(2n)^k}^(2n-2r-1)/2n
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sinx=sinx/(2n+1)・Σ(−1)^r/r!Π(2n−k)(2cosx/(2n+1))^2n-2r
=(2n+1)sinx/(2n+1){Σ(−1)^r/r!Π(2n−k)(2cosx/(2n+1))^2n-2r}/(2n+1)
したがって,
sinx/x=Π{Σ(−1)^r/r!Π(2n−k)(2cosx/(2n+1)^2n-2r}/(2n+1)
=ΠΣ(−1)^r(2n+1,2r+1){sinx/(2n+1)^k}^2r{cosx/(2n+1)^k}^(2n-2r)}/(2n+1)
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