## anonymous one year ago 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

1. anonymous

@dan815

2. anonymous

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

3. anonymous

let a be the first term find a2 and a5

4. anonymous

b? @surjithayer

5. anonymous

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

6. anonymous

can you find?

7. anonymous

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

8. SolomonZelman

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))

9. anonymous

@SolomonZelman its c right?

10. SolomonZelman

yes, it's C.