巧算2分之1加6分之1加十二分之1加三十分之1加四十二分之1

1/2 +1/6 +1/12 +1/30 +1/42=1/2+1/(2*3)+1/(3*4)+1/(5*6)+1/(6*7)=1/2+1/2-1/3+1/3-1/4+1/5-1/6+1/6-1/7=1-1/4+1/5-1/7=(140-35+28-20)/140=113/140