Posted by **Jon** on Thursday, December 13, 2007 at 2:55pm.

1)Find a1 in a geometric series for which Sn=300,r=-3,and n=4

A)15

B)15/2

C)-15

D)1/15

I chose A

5)Find the fifth term of the sequence in which a1=-3,and aN+1=3aN-n

A)-301

B)-99

C)-193

D)-341

I don't know

- math -
**Reiny**, Thursday, December 13, 2007 at 5:15pm
Jon, for #1 the equation is

300 = a((-3)^4 - 1)/(-3-1)

300 = a(80)/-4

a = -15

since there are only 4 terms, this is easy to check

-15 + 45 + (-135) + 405 = 300

for you #2, I will assume you meant

a_{1} = -3

a_{N+1} = 3_{N} - n

then:

a_{1} = -3

a_{2} = 3a_{1} - 2 = -11

a_{3} = 3a_{2} - 3 = -36

a_{4} = 3a_{3} - 4 = -112

a_{5} = 3a_{4} - 5 = -341 which is choice D

- math -
**Isabel**, Tuesday, July 22, 2014 at 3:03pm
find a1 for given geometric series.

Sn= 405, r=3, n=4

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