1*3+2*4+3*5+4*6+``````19*21+20*22=
问题描述：
1*3+2*4+3*5+4*6+``````19*21+20*22=
答
1*3+2*4+3*5+4*6+``````19*21+20*22
=4-1 +9-1 +16-1 +...+400-1 +441-1
=1²+2²+3²+...+21² -21
=21*（21+1）*（2*21+1）/6 -21
=3311-21
=3290