If $2, x, 20$ are three consecutive terms of an $A.P.$,then $x = ..........$

Let $S_n$,$S_{2n}$,and $S_{3n}$ be the sums of $n$,$2n$,and $3n$ terms of an Arithmetic Progression $(AP)$,respectively. Prove that $S_{3n} = 3(S_{2n} - S_n)$.

Find the sum of the two middle most terms of the $AP: -\frac{4}{3}, -1, -\frac{2}{3}, \ldots, 4 \frac{1}{3}$.

Find the sum:
$4-\frac{1}{n}+4-\frac{2}{n}+4-\frac{3}{n}+\ldots$ up to $n$ terms.

Find the $12^{\text{th}}$ term from the end of the $AP: -2, -4, -6, \ldots, -100$.

How many terms of the $A.P.$ $8, 15, 22, \ldots$ add up to $1490$?