x2=5+4=9

x 1 '-x2 ' = 1 1-9 = 2。

A: Four years later, Bingbing is two years older than Lili.

Method 2: Suppose that after N years, n:N*.

x 1'=x 1+n=7+n

x2'=x2+n=5+n

X1'-x2' = (7+n)-(5+n) = 7+n-5-n = 7-5 = 2 (constant).

Prove that no matter how many years have passed, the age difference between the two is a custom 2.

N=4 belongs to N*,

To further complicate this conclusion,

For four years, Bingbing is two years older than Lili.