整体代换
设t=a2+...an-1
(a1+a2+...an-1)(a2+a3+...an-1+an)-(a2+a3+...+an-1)*(a1+a2+...+an)
=(a1+t)(t+an)-t(a1+t+an)
=t^2+(a1+an)t+a1*an-(a1+an)t-t^2
=a1*an