5/6-7/12-9/20-11/30-13/42-15/56-17/72-19/90
=(1/2+1/3)-(1/3+1/4)-(1/4+1/5)-(1/5+1/6)-(1/6+1/7)+(1/7+1/8)+(1/8+1/9)-(1/9+1/10)
=1/2+(1/3-1/3)+(1/4-1/4)+(1/5-1/5)+(1/6-1/6)+(1/7-1/7)+(1/8-1/8)+(1/9-1/9)+1/10
=1/2+1/10
=3/5
5/6-7/12-9/20-11/30-13/42-15/56-17/72-19/90
=(1/2+1/3)-(1/3+1/4)-(1/4+1/5)-(1/5+1/6)-(1/6+1/7)+(1/7+1/8)+(1/8+1/9)-(1/9+1/10)
=1/2+(1/3-1/3)+(1/4-1/4)+(1/5-1/5)+(1/6-1/6)+(1/7-1/7)+(1/8-1/8)+(1/9-1/9)+1/10
=1/2+1/10
=3/5