To find $ a_{ 55 } $ we use formula

$$ \color{blue}{a_n = a_1 + (n-1)d}$$

In this example we have $ a_1 = 8,~~ d = 16 ~~,~~ n = 55 $. After substituting these values into the formula, we obtain:

$$ \begin{aligned} a_n &= a_1 + d(n-1) \\[1 em] a_{ 55 } &= 8 + (55-1) \cdot 16 \\[1 em] a_{ 55 } &= 8 + 864 \\[1 em] a_{ 55 } &= 872 \end{aligned} $$

The first few terms of this sequence are:

$$ 8, ~~~24, ~~~40, ~~~56, ~~~72 . . . $$

Arithmetic sequences