利用两个公式
1+2+3+.+n=n(n+1)/2
1²+2²+3²+.+n²=n(n+1)(2n+1)/6
1x2+2x3+3x4+...+100x101
=1x1+2x2+3x3+...+100x100+(1+2+3+.+100)
=100x101x201/6+(1+100)*100/2
=343400