7/12=(3+4)/3*4=1/3+1/4
9/20=(4+5)/4*5=1/4+1/5
...
13/42=(6+7)/6*7=1/6+1/7
原式=1+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7-45/56
=1+8/56-45/56
=19/56