几道初中计算题-1/10乘11 - 1/11乘12 - 1/12乘13 .- 1/19乘20-1/2 - 1/6 - 1

-1/10乘11 - 1/11乘12 - 1/12乘13 .- 1/19乘20
=-(1/10-1/11+1/11-1/12+.+1/19-1/20)
=-(1/10-1/20)
=-1/2
-1/2 - 1/6 - 1/12 - 1/20 - 1/30 - 1/42
=-(1-1/2+1/2-1/3+1/3-1/4+...+1/6-1/7)
=-(1-1/7)
=-6/7