To find $ a_{ 30 } $ we use formula

$$ \color{blue}{a_n = a_1 \cdot r^{n-1}}$$

In this example we have $ a_1 = 1 ~~,~~ r = 2 ~~\text{and}~~ n = 30 $. After substituting these values to above formula, we obtain:

$$ \begin{aligned} a_n &= a_1 \cdot r^{n-1} \\ a_{ 30 } &= 1 \cdot 2^{ 30 - 1} \\ a_{ 30 } &= 1 \cdot 536870912 \\ a_{ 30 } &= 536870912 \end{aligned}$$

The first few terms of this sequence are:

$$ 1, ~~~2, ~~~4, ~~~8 . . . $$

Geometric sequences