Write the explicit formula for the geometric sequence. Then find the fifth term in the sequence.

$a_1 = –4,~a_2 = 8,~a_3 = –16$

A) $a_n = –4 · (2)^n; –64$
B) $a_n = –4 · (–2)^{n–1}; –64$
C) $a_n = –4 · (–2)^n; 128$
D) $a_n = –2 · (–4)^{n–1}; –512$

Question

Write the explicit formula for the geometric sequence. Then find the fifth term in the sequence.

$a_1 = –4,~a_2 = 8,~a_3 = –16$

A)    $a_n = –4 · (2)^n; –64$
B)    $a_n = –4 · (–2)^{n–1}; –64$
C)    $a_n = –4 · (–2)^n; 128$
D)    $a_n = –2 · (–4)^{n–1}; –512$

campbell_st · Answer

well the general term in a geometric sequence is 

$$a_{n} =a_{1} \times r^{n -1}$$

you know the 1st term.... you need to common ratio r..... before you can find the 5th term 

hope it helps

anonymous · Answer

Using what you said would B be correct?
$a_{n} =a_{1} \times r^{n -1}$
$a_{n} =-4 \times -2^{n -1}$

campbell_st · Answer

yeap...thats it, you now just need to substitute n = 5 and then evaluate