9. (5 points) Xiaoming rides a bike to visit three scenic spots $A, B, C$. If he starts from $A$ and goes through $B$ to $C$, he travels a total of 10 kilometers; if he starts from $B$ and goes through $C$ to $A$, he travels a total of 13 kilometers; if he starts from $C$ and goes through $A$ to $B$, he travels a total of 11 kilometers, then the distance between the two closest scenic spots is $\qquad$ kilometers.
【Solution】Solution: Let the distance between $A$ and $B$ be $x$ kilometers, the distance between $B$ and $C$ be $y$ kilometers, and the distance between $A$ and $C$ be $z$ kilometers. According to the problem, we can set up the following system of equations:
$$
\left\{\begin{array}{l}
x+y=10 \\
y+z=13 \\
z+x=11
\end{array}\right.
$$
(2) - (1) gives: $z-x=3$, (4),
(3)+(4) gives: $2 z=14$, so $z=7$,

Substituting $z=7$ into (2) gives: $y=6$,
Substituting $z=7$ into (3) gives: $x=4$,
Therefore, the solution to this system of equations is: $\left\{\begin{array}{l}x=4 \\ y=6 \\ z=7\end{array}\right.$
Answer: The shortest distance between two scenic spots is 4 kilometers.
Therefore, the answer is: 4.