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

a1 = -4, a2 = 8, a3 = -16

an=-4*(2)^n; -64
an=-4*(-2)^n-1; -64

Question

Write the explicit formula for the geometric sequence.  Then find the fifth term in the sequence.  

a1 = -4, a2 = 8, a3 = -16

an=-4*(2)^n; -64
an=-4*(-2)^n-1; -64

anonymous · Answer

What do you think the answer is?

janu16 · Answer

Just multiply each new term by -2, so a2 = a1 * -2, and so on:

As a result a4: 32 and a5 = -64

liv1234 · Answer

I was able to narrow it down to those two options, but can't figure out which one is right..

janu16 · Answer

example is up there^^

janu16 · Answer

multiply each new termby -2

liv1234 · Answer

I know, but I need help... I already saw the example from another page

liv1234 · Answer

@haleyelizabeth2017

liv1234 · Answer

@jigglypuff314

liv1234 · Answer

@Michele_Laino

anonymous · Answer

Okay, so we've established the common ratio is -2. We also have the first time, $a_1=-4$. Now we can use these in the formula for a geometric sequence:
$$a_n=a_1*r^{n-1}=-4*-2^{n-1}$$To find the fifth term, we set n=5:$$a_5=-4*-2^{5-1}=64$$