20^2+21^2+22^2++50^2
答案:20^2+21^2+22^2++50^2
=(1^2+2^2+3^2++50^2)-(1^2+2^2+3^2++19^2)
=50(50+1)(250+1)6-19(19+1)(219+1)6
=42925-2470
=40455