Answer
$${S_{120}} = 7260$$
Work Step by Step
$$\eqalign{
& {\text{The first }}120{\text{ positive integers form the arithmetic sequence }} \cr
& {\text{1}},{\text{ 2}},{\text{ 3}},{\text{ 4}},{\text{ }}.{\text{ }}.{\text{ }}.{\text{ }},{\text{ }}120. \cr
& {\text{Let }}{a_1} = 1,\,\,{a_{120}} = 120,\,\,n = 120 \cr
& {\text{Using the formula }}{S_n} = \frac{n}{2}\left( {{a_1} + {a_n}} \right) \cr
& {S_{120}} = \frac{{120}}{2}\left( {{a_1} + {a_{120}}} \right) \cr
& {S_{120}} = 60\left( {1 + 120} \right) \cr
& {S_{120}} = 7260 \cr} $$
