cosx = 1 - x^2 / 2 + o(x^2)(cosx)^n = 1 - nx^2 / 2 + o(x^2)那么 lim (cosx + .... + (cosx)^n - n) / (cosx - 1) = (-1/2 + -2/2 + ... + -n/2) / (-1/2) = n(n+1)/2
