3又1/2*(-5/7)-(-5/7)*2又1/2-5/7*(-1/2)=5/7(-7/2+5/2+1/2)=5/7*(-1/2)=-5/14
3又1/2*(-5/7)-(-5/7)*2又1/2-5/7*(-1/2) =5/7×(-3又1/2+2又1/2+1/2)=5/7×(-1/2)=-5/14
-5/14