6分之5-12分之7+20分之9-30分之11+42分之13-56分之26+72分之17-90分之19=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...