因为(2x-1)/(3-4x)=-[(2x-1)/(3-4x)]大于等于0,
故必须有(2x-1)/(3-4x)=-[(2x-1)/(3-4x)]=0
得x=1/2,y=0+0+1/2=1/2
5x-3y=5/2-3/2=1