Write the explicit formula for the geometric sequence.
a1 = -5 a2 = 20 a3 = -80

A. an = -5 • (-4)n
B. an = -5(-4)n-1
C. an = -4(-5)n-1
D. an = -5 • (4)n

Question

Write the explicit formula for the geometric sequence.
a1 = -5  a2 = 20  a3 = -80

A. an = -5 • (-4)n
B. an = -5(-4)n-1
C. an = -4(-5)n-1
D. an = -5 • (4)n

bahrom7893 · Answer

look
a1 = -5
n=1

a2=20
n=2, etc..

jhannybean · Answer

It would be B because in a geometric sequence, you have $$\large ar ^{n-1}$$ and in finding our r, we take $\large\frac{ a _{2} }{ a _{1} }$ and that is -4

bahrom7893 · Answer

This way you can eliminate A because term 2 doesn't match up... And I think Jhanny's right

anonymous · Answer

thanks you that was heard

jhannybean · Answer

So we have a1 = -5, our r = -4  and plug it into our formula $$\large ar ^{n-1}$$

jhannybean · Answer

thanks bahrom :)

anonymous · Answer

Can you help me with another

jhannybean · Answer

sure

anonymous · Answer

What is the simplified form of the expression?|dw:1369539526178:dw|