2分之1+6分之1+12分之1+20分之1+30分之1+42分之1+56分之1
=6/12+2/12+1/12+3/60+2/60+1/42+1/56
=9/12+5/60+1/42+1/56
=10/12+1/42+1/56
=5/6+1/42+1/56
=35/42+1/42+1/56
=36/42+1/56
=6/7+1/56
=48/56+1/56
=49/56
=7/8