To find the missing denominator in the addition expression \( \frac{8}{11} + \frac{1}{?} \), we need the two fractions to have a common denominator.
Let's denote the missing denominator by \( x \). To find the sum, we need both fractions to have the same denominator:
\[ \frac{8}{11} + \frac{1}{x} = \frac{8x + 11}{11x} \]
For the addition to be valid, we can set \( x \) equal to 11 to maintain the common denominator.
Thus, with a common denominator of 11, we have:
\[ \frac{8}{11} + \frac{1}{11} = \frac{8 + 1}{11} = \frac{9}{11} \]
So, the missing denominator \( x \) is 11.
The answer is:
11