x2-y2=(x+y)(x-y),20x20-19x19=1*39=39,18x18-17x17=35*1=35......20x20-19x19+18x18-17x17+…+2x2-1x1=39+35+。。悉野手。。睁嫌脊镇。+3=210
用平方差公式,化简得:20+19+18。。。。+2+1=21*20/2=210
A