解:(1/4+1/5+1/6+1/7)^2-(1/4+1/5+1/6+1/7)*(1/5+1/6+1/7) =(1/4+1/5+1/6+1/7)(1/4+1/5+1/6+1/7-1/5-1/6-1/7) =309/420*1/4 =309/1680