sin(X)/x的幂级数
问题描述：
sin(X)/x的幂级数
答
sin(x) = x - x^3/3! + x^5/5! - x^7/7! + .
sin(x)/x = 1- x^2/3! + x^4/5! - x^6/7!+.
sin(x)/x =Σ_(n=0)^(∞)(-1)^n x^2n / (2n+1)!