1/(19×21)+1/(21×23)+1/(23×25）+……+1/(97×99)
原式=1/2×(1/19-1/21)+1/2×(1/21-1/23)+1/2×(1/23-1/25)+……1/2×(1/97-1/99)
=1/2×(1/19-1/21+1/21-1/23+1/23-1/25+1/25-1/27+……+1/95-1/97+1/97-1/99）
=1/2(1/19-1/99)
=40/1881
我想知道一下原式的第一步和第二步怎么简化的呢?