解:原式=(1/3+1/4+1/5+1/6)×(1/4+1/5+1/6+1/7)-(1/3+1/4+1/5+1/6)-(1/4+1/5+1/6+1/7)+1-1=[(1/3+1/4+1/5+1/6)-1]×[(1/4+1/5+1/6+1/7)-1]-1=(1/30)×(-101/420)-1=-101/12600-1=-12701/12600.