9. (10 points) There are 11 square formations, each consisting of the same number of soldiers. If 1 general is added, a large square formation can be formed. The minimum number of soldiers in one of the original square formations is $\qquad$.


【Analysis】This problem examines the issue of square arrays.
【Solution】Solution: According to the problem, let the original square array have $a$ soldiers, then $a$ and $11 a+1$ are perfect squares,

When $a=1$, $11 a+1=12$, which does not meet the requirement;
When $a=4$, $11 a+1=45$, which does not meet the requirement;
When $a=9$, $11 a+1=100$, which meets the requirement,
Therefore, the original square array must have at least 9 soldiers.