Answer
$\displaystyle \sum_{n=1}^{7}\frac{n}{n+3}$
Work Step by Step
There are seven terms (we count through numerators, starting with 1)
( $\displaystyle \sum_{n=1}^{7}a_{n}$ )
in which the numerator of the n-th term is $n,$ and the denominator is $n+3$.
So, $a_{n}=\displaystyle \frac{n}{n+3}$
Sum = $\displaystyle \sum_{n=1}^{7}\frac{n}{n+3}$
