1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90
=1-1/2+1/2-1/6+1/6-1/12+1/12-1/20+1/20-1/30+1/30-1/42+1/42-1/56+1/56-1/72+1/72-1/90
=1-1/90
=89/90