2分之1+6分之1+12分之1+20分之1+30分之1
=½+(½-1/3)+(1/3-¼)+(¼-1/5)+(1/5-1/6)
=½+½-1/3+1/3-¼+¼-1/5+1/5-1/6
=½+½-1/6
=1-1/6
=5/6