Question

find an equation for the nth term of a geometric sequence where the second and fifth terms are -21 and 567, respectively.

an = 7 • (-3)n^( + 1)
an = 7 • 3^(n - 1)
an = 7 • (-3)^(n - 1)
an = 7 • 3^n

Answer 1

@dan815

Answer 2

\[a _{n}=ar ^{n-1^{}}\]
\[find~a _{2}~and~a _{5}\]
and divide

Answer 3

let a be the first term
find a2 and a5

Answer 4

b? @surjithayer

Answer 5

\[a _{2}=ar ^{2-1}=ar=-21\]
\[a _{5}=ar ^{5-1}=ar^4=567\]
divide
\[\frac{ ar^4 }{ ar }=\frac{ 567 }{ -21 },r^3=-27=\left( -3 \right)^3,r=-3\]
ar=-21
find a

Answer 6

can you find?

Answer 7

\[\frac{ ar }{ r }=\frac{ -21 }{ -3 }=?\]
then write an=?

Answer 8

if 2nd term is negative and 5th term is positive,
then the common ratio is obviously negative.

((There is no other way for such geom. sequence))

Answer 9

@SolomonZelman its c right?

Answer 10

yes, it's C.