1/6-1/12-1/20-1/30-1/42-1/56=(1/2-1/3)-(1/3-1/4)-(1/4-1/5)-(1/5-1/6)-(1/6-1/7)-(1/7-1/8)=1/2-1/3-1/3+1/8=-1/24
1/6-1/12-1/20-1/30-1/42-1/56=1/12-1/12-1/42-1/56=(-8-6)/6*7*8
