一道数学题：5/6-7/12+9/20-11/30+...+{18+19}/18X19-{19+20}/19X20谢谢了,

5/6-7/12+9/20-11/30+...+{18+19}/18X19-{19+20}/19X20 = (1/2 + 1/3) - (1/3 + 1/4) + (1/4 + 1/5) - (1/5 + 1/6) + .+ (1/18 + 1/19) - (1/19 + 1/20) = 1/2 - 1/20 = 9/20