Question

In the geometric progression 1, 2, 4… what term is 512?

Answer 1

The next term is double the previous term, so that the $n$-th term is given recursively by

$\begin{cases}a_1=1\\a_n=2a_{n-1}&\text{for }n>1\end{cases}$

This rule tells us that

$a_2=2a_1$

$a_3=2a_2=2^2a_1$

$a_4=2a_3=2^3a_1$

and so on, with the explicit rule

$a_n=2^{n-1}a_1=2^{n-1}$

for $n\ge1$.

If 512 is the $k$-th term in the sequence, then

$512=2^{k-1}\implies\log_2512=\log_22^9=\log_22^{k-1}\implies9=k-1\implies k=10$