SOLUTION: find the 9th term of a geometric progression whose 4th and 7th terhs are -4 and 256 respectively

Algebra ->  Sequences-and-series -> SOLUTION: find the 9th term of a geometric progression whose 4th and 7th terhs are -4 and 256 respectively      Log On


   

Question 751099: find the 9th term of a geometric progression whose 4th and 7th terhs are -4 and 256 respectively
Answer by stanbon(75887) About Me  (Show Source):
You can put this solution on YOUR website!
find the 9th term of a geometric progression whose 4th and 7th terhs are -4 and 256 respectively
----
Equations:
ar^(6) = 256
ar^(3) = -4
----
Divide to get:
r^3 = -64
r = -4
--------
Solve for "a":
ar^3 = -4
a(-64) = -4
a = 1/4
------
Equation:
a(n) = (1/4)(-4)^(n-1)
a(9) = (1/4)(-4)^8 = (1/4)65536 = 16384
===============
Cheers,
Stan H.
===============