9. (8 points) There are 10 cards, each with a number written on it: $1,2,3, \cdots, 9,10$. How many cards must be drawn to ensure that among the drawn cards, there are two cards whose numbers add up to 11?
【Answer】Solution:
$$
\begin{array}{l}
10 \div 2=5 \text { (pieces) } \\
5+1=6 \text { (pieces) }
\end{array}
$$

So the answer is 6