-2,_____,______,54; geo sequence

• Oct 27th 2007, 02:43 PM
beetz
-2,_____,______,54; geo sequence
and I don't know how to do this one either:
1/4,____,4

Or this one:
_____,_______,_______,2.5 r=2

My textbook doesn't go over this either....

Thanks
• Oct 27th 2007, 03:23 PM
Jhevon
Quote:

Originally Posted by beetz
and I don't know how to do this one either:
1/4,____,4

recall that the terms of a geometric series are given by:

$\displaystyle a_n = ar^{n - 1}$ for $\displaystyle n = 1,2,3,4...$

where $\displaystyle a_n$ is the nth term, $\displaystyle a$ is the first term, $\displaystyle r$ is the common ratio, and $\displaystyle n$ is the current number of the term.

here we see that $\displaystyle a = \frac 14$ and $\displaystyle a_3 = ar^2 = \frac 14r^2 = 4$

solve for $\displaystyle r$ and you can find the middle term.

Quote:

Or this one:
_____,_______,_______,2.5 r=2

My textbook doesn't go over this either....

Thanks
here you have $\displaystyle a_4 = 2.5 = ar^3 = 8a$

solve for $\displaystyle a$ and you can find the other terms