若x>=1,y>=1,则（x-1)(y-1)=xy-x-y+1>=0,
∴xy>=x+y-1.于是
若实数a1,a2,...,a8均不小于1 ,
则a1a2a3a4,a5a6a7a8不小于1,
∴a1a2…a8>=a1a2a3a4+a5a6a7a8-1,
同理a1a2a3a4>=a1a2+a3a4-1,
a5a6a7a8>=a5a6+a7a8-1,
则a1a2…a8>=a1a2a3a4+a5a6a7a8-1>=a1a2+a3a4-1+a5a6+a7a8-1-1>=a1a2+a3a4+a5a6+a7a8-3
同理：
a1a2>=a1+a2-1,
a3a4>=a3+a4-1,
a5a6>=a5+a6-1,
a7a8>=a7+a8-1,
∴a1a2…a8>=a1+a2+a3+a4+a5+a6+a7+a8-7=13,
这与“a1a2...a8