#### Answer

$4.9n^2$.

#### Work Step by Step

The sum of the first $n$ terms of an arithmetic sequence can be obtained by the following formula: $\frac{n(a_1+a_n)}{2},$ where $a_1$ is the first term, $a_n$ is the nth term and $n$ is the number of terms.
The nth term of an arithmetic sequence can be obtained by the following formula: $a_n=a_1+(n-1)d$, where $a_1$ is the first term and $d$ is the common difference.
hence $S_n=\frac{n(a_1+a_n)}{2}=\frac{n(a_1+a_1+(n-1)d)}{2}$.
We know that $a_1=4.9,d=9.8$. Thus $\frac{n(4.9+4.9+(n-1)9.8)}{2}=\frac{n(9.8+9.8n-9.8)}{2}=\frac{9.8n^2}{2}=4.9n^2$.