21^2+22^2+……+50^2
=(1^2+2^2+...+50^2)-(1^2+2^2+...+20^2)
=50(50+1)(2*50+1)/6 -20(20+1)(2*20+1)/6
=42925-2870
=40055
注意:1^2+2^2+3^2+...+n^2=n(n+1)(2n+1)/6