Answer
$9 \sqrt 2$
Work Step by Step
The $n^{th}$ term of an arithmetic sequence is given by the formula:
$a_n = a_1 + (n-1) \ d(1)$
where, $a_1 = \ First \ Term$ and $ d = \ Common \ Difference$
We have: $a_1=\sqrt 2$ and $ d=a_2-a_1=2 \sqrt 2-\sqrt 2=\sqrt 2$
Plug in $9$ for $n$ into equation (1) to obtain:
$a_{9} =\sqrt 2+(9-1) (\sqrt 2) =9 \sqrt 2$
