11. (7 points) On the blackboard, there are several consecutive odd numbers starting from 1: $1, 3, 5, 7, 9, 11, 13 \cdots$ After erasing one of the odd numbers, the sum of the remaining odd numbers is 1998. What is the odd number that was erased? $\qquad$
【Solution】Solution: The sum of the odd number sequence from 1 to $2 n-1$ is: $(1+2 n-1) \times n \div 2=n^{2}>1998$,

Also, $44^{2}=1936<1998$;
Thus, $n=45$, the odd number that was subtracted is $2025-1998=27$. Therefore, the answer is: 27.