50×50-49×49＋48×48-……+2×2-1×1=50×50-（50-1）×49+……2×2-（2-1）×1=50×50-50×49+49+……+2×2-2×1+1×1=50+49+……+2+1(50-49)*(50+49)+(48-47)*(48+47)+(46-45)*(46+45)+.....+(2-1)*(2+1)根据这一步怎么的出来啊