The nth Term

The list 2, 5, 8, 11, ... adds 3 each time. A list that changes by the same amount at every step is an arithmetic sequence, and the amount is its common difference d. Here the first term is 2 and d is 3.

To reach the 10th term you do not need to write out ten numbers. Starting from 2, the 10th term is 9 steps along, because the first term took no steps at all. So it is $2+9\cdot 3$, which is 29. In general, with a(1) the first term, the term in position n is

$$a\left(n\right) = a\left(1\right)+\left(n-1\right)\cdot d$$

Written as a function, the rule for 2, 5, 8, ... is $a\left(n\right) = 2+\left(n-1\right)\cdot 3$, and $a\left(10\right)$ is its 10th term.