1/1x2= 1-1/21/2x3=1/2-1/3依此类推原式 = 1-1/2+1/2-1/3+1/3-...+1/19-1/20 =1-1/20 =19/20
1-1/2+1/2-1/3+1/3-...+1/19-1/20=1-1/20 =19/20
