7、A book's page numbers are consecutive natural numbers, $1,2,3, \cdots$, when these page numbers are added together, one page number is added twice, resulting in the incorrect sum of 1997, then the page number that was added twice is $\qquad$
7. Solution: The page numbers of this book are natural numbers from 1 to $n$, and their sum is $1+2+\cdots+n=\frac{n(n+1)}{2}$. The incorrectly added page number is between 1 and $n$, meaning 1997 should be between $\frac{n(n+1)}{2}+1$ and $\frac{n(n+1)}{2}+n$.
When $n=61$, the sum is $1891, \frac{n(n+1)}{2}+n=1891+61=19521997$, which does not meet the requirement.
Therefore, $n=62$, the correct sum is 1953, and the incorrectly added page number is $1997-1953=44$