Analysis: In this problem, it can be assumed that one computer will infect X computers in each round of infection, so * * computers will be infected after the first round, and * * computers will be infected after the second round, that is, (1+x)+x (1+x).

Solution: We assume that each computer will infect X computers in each round of infection, that is:1+x+(1+x) x = 81.

Arrangement (1+x)2=8 1,

Then x+ 1=9 or x+ 1=-9,

The solution is x 1=8, x2=- 10 (abbreviated),

∴( 1+x)3=( 1+8)3=729>700.

A: Each computer will infect 8 computers in each round of infection. After three rounds of infection, more than 700 computers will be infected.