Answer
$\sum_{i=2}^{21} \frac{i}{i+1}$
Work Step by Step
As we can see the numerators start at $2$ and increase by $1$ at each term until they reach $21$, the denominators start at $3$ and increase by $1$ at each term until they reach $22$, thus the formula is: $\sum_{i=2}^{21} \frac{i}{i+1}$
