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
=1/2+(1/2+1/3)-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)
=1/2+1/2+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7
=1/2+1/2+1/7
=1+1/7
=8/7