sinx/x=(1-x^2/π^2)(1-x^2/4π^2)(1-x^2/9π^2)・・・に対して,
sin2x/2x=sinx/x・cosx
より
cosx=(1-4x^2/π^2)(1-4x^2/9π^2)(1-4x^2/25π^2)
===================================
(1-4/1)(1-4/9)(1-4/25)・・・(1-4/n^2)
=(-1・3/1・1)・(1・5/3・3)・(3・7)/(5・5)・・・(2n-3)(2n+1)/(2n-1)(2n-1)
=-(2n+1)/(2n-1)→-1
同様に
(1-16/1)(1-16/9)(1-16/25)(1-16/49)・・・=1
===================================