分类讨论呗1。xy=a+b+c+d-4x>b+c+d-3a2.a<=xy=b-a+c+d-2x>c+d-b-a3.b<=xy=c+d-a-b4.c<=xy=2x+d-c-b-a>=d+c-b-a5.d<=xy=4x-a-b-c-d>=3d-a-b-c而c+d-a-b<3d-a-b-c所以最小值为c+d-a-b,此时b<=x<=c
