1x2/1+2x3/1+3x4/1+4x5/1.19x20/1

题目

1x2/1+2x3/1+3x4/1+4x5/1.19x20/1

答案

=2x(2-1)+3x(3-1)+4x(4-1)+.+20x(20-1)
=2^2+3^2+4^2+.+20^2-(2+3+4+.+20)
因为1^2+2^2+3^2+...+n^2=n(n+1)(2n+1)/6
所以=2869-209
=2660