cosx=1-x^2/2!+x^4/4!-x^6/6!+...xcosx=x-x^3/2!+x^5/4!-x^7/6!+....
cosx=∑(n=0→∞)(-1)^n·x^(2n)/(2n)!【x∈(-∞，+∞)】∴xcosx=∑(n=0→∞)(-1)^n·x^(2n+1)/(2n)!【x∈(-∞，+∞)】
cosx=1-1/2x^2+1/4!x^4
