1-1／2+1／6+1／12+1／20+1／30+...+1／72+1／90

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