Answer
$a_n=n(n+2)$; $n=1,2,3,4,....$
Work Step by Step
Since, we have $1 \cdot 3,2 \cdot 4,3 \cdot 5,4 \cdot 6...$
or, $1 \cdot (1+2),2 \cdot (2+2),3 \cdot (3+2),4 \cdot (4+2)...$
This sequence shows an arrangement of alternate signs.
which can be represented as:
$a_n=n(n+2)$; $n=1,2,3,4,....$
