20*20-19*19+18*18-17*17+...+2*2-1*1
平方差
=(20-19)(20+19)+……+(2-1)(2+1)
=1*(20+19)+……+1*(2+1)
=20+19+18+17+……+2+1
=(20+1)+(19+2)+……+(11+10)
=21*10
=210