1/2=1-1/21/6=1/2-1/31/12=1/3-1/41/20=1/4-1/51/30=1/5-1/61/42=1/6-1/7……1/90=1/9-1/101／2+1／6+1／12+1／20+1／30+1／42+1／56+1／72+1／90=（1-1/2）+（1/2-1/3）+(1/3-1/4)+……+（1/9-1/10）=1-1/10=9/10...