# Thread: arithmetic or geometric?

1. ## arithmetic or geometric?

State the common difference or ratio and determine a formula for the nth term.

5,10,20,40,...

I got the common difference which is X 2. But I dont get the 2nd part.

2. Originally Posted by blame_canada100
State the common difference or ratio and determine a formula for the nth term.

5,10,20,40,...

I got the common difference which is X 2. But I dont get the 2nd part.

The nth term of a geometric sequence with initial value a and common ratio r is given by ${a_n} = r \times a^{n-1}$

So,
The nth term for this sequence would be given by:

${a_n} = 5 \times 2^{n-1}$

3. Originally Posted by blame_canada100
State the common difference or ratio and determine a formula for the nth term.

5,10,20,40,...

I got the common difference which is X 2. But I dont get the 2nd part.
$a_n=a_1r^{n-1}$
$a_n=5(2)^{n-1}$