1又二分之一-六分之五+十二分之七-二十分之九+三十分之十一-四十二分之十三
=1+1/2-5/6+7/12-9/20+11/30-13/42
=1+1/2-2/6-3/6+3/12+4/12-4/20-5/20+5/30+6/30-6/42-7/42
=1+1/2-1/3-1/2+1/4+1/3-1/5+1/4+1/6+1/5-1/7-1/6
=1-1/7
=6/7