f(x) = e^(-x^2)considere^x = 1+x/1! +x^2/2!+.....+ x^n/n! +...x= -x^2e^(-x^2) = 1-x^2/1! +x^4/2!+.....+ (-1)^n .x^(2n)/n! +...
就是求导两次呀。
