1又3/4=7/4,0.875=7/8,-1又1/7=-8/姿族7(1又3/4-0.875-7/12)乘瞎闷(-1又1/7)= (7/4-7/8-7/12)*(-8/7)=7*(-8/7)*(1/4-1/8-1/12)=-8*(1/迹神弊4-1/8-1/12)=-2+1+8/12=-1/3