x≤a,y≤b,z≤cx\le a,y\le b,z\le cx≤a,y≤b,z≤c
求证:
3abc+4ayz+4bxy+4cxz−4bcx−4aby−4ca>=03abc+4ayz+4bxy+4cxz-4bcx-4aby-4ca>=03abc+4ayz+4bxy+4cxz−4bcx−4aby−4ca>=0