9 9r 9r^2 9r^3
-72 = 9 r^3
r^3 = -72/9
now work back
a9 = 9/r
a8 = 9/r^2
a7 = 9/r^3
interesting
a7 = 9/r^3 = (9)(-9/72) = -81/72
what is the 7th geometric sequence when a10 is 9 and a13 is -72
2 answers
or ,
using the standard a to be first term, r the common ratio
ar^12 = -72
ar^9 =9
divide them
r^3 = -8
r = -2 , then from ar^9 = 9
a = 9/(-2)^9 = -9/512
term(7) = ar^6
= (-9/512)(-2^6) = -576/512 = -9/8
using the standard a to be first term, r the common ratio
ar^12 = -72
ar^9 =9
divide them
r^3 = -8
r = -2 , then from ar^9 = 9
a = 9/(-2)^9 = -9/512
term(7) = ar^6
= (-9/512)(-2^6) = -576/512 = -9/8
1 answer