12分之2加20分之2加30分之2加42分之2加56分之2加72分之2加90分之2=2×(1/12+1/20+1/30+1/42+1/56+1/72+1/90)=2×(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)=2×(1/3-1/10)=7/15...