$fracnn+12$

This expression represents the sum of the first $n$ positive integers. It is also known as the $n$th triangular number, because if you represent each integer as a dot and arrange them in a triangular pattern, this expression gives you the total number of dots. For example, the fourth triangular number is $\frac{4(4+1)}{2}=10$, which you can represent as a triangle with four rows:

$$ \begin{matrix} \cdot \ \cdot \quad \cdot \ \cdot \quad \cdot \quad \cdot \ \cdot \quad \cdot \quad \cdot \quad \cdot \end{matrix} $$