Asked by Jon

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
Answered by Reiny
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<sub>1</sub> = -3
a<sub>N+1</sub> = 3<sub>N</sub> - n

then:
a<sub>1</sub> = -3
a<sub>2</sub> = 3a<sub>1</sub> - 2 = -11
a<sub>3</sub> = 3a<sub>2</sub> - 3 = -36
a<sub>4</sub> = 3a<sub>3</sub> - 4 = -112
a<sub>5</sub> = 3a<sub>4</sub> - 5 = -341 which is choice D
Answered by Isabel
find a1 for given geometric series.

Sn= 405, r=3, n=4