math
posted by Jon on .
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
2)Find the sum of the infinite geometric series. Sigma sign with infinity symbol above and n=1 below. To the right 20(1/4)n1
A)25
B)80/3
C16
D)does not exist
I chose A
3)Find the sum of the infinite geometric series:4+3+9/4+:...
A)16/7
B)16
C)12
D)does not exist
I chose B
4)Write 0.72 repeating as a fraction.
A)7/9
B)8/11
C)18/25
D)7 and 2/9
I chose B but my book confuses me on how to work it out. I just divided 8 and 11 and I got .72 repeating
5)Find the fifth term of the sequence in which a1=3,and aN+1=3aNn
A)301
B)99
C)193
D)341
I don't know

1. I chose B
2. C
3. Hey, I better start showing you how
1. Plain geometric series
Sn = g(1r^n)/(1r)
300 = g (13^4) /13
300 = g (181) / 2
g = 300 (2/80)
g = 15/2
2. sigma = g/(1r)
g=20
r=1/4
sigma = 20/1.25
= 2000/125
= 400/25
= 16
3. g = 4
r = 3/4
s = 4/(13/4)
=16
4. .72 72 72 72 ....
= 72 10^2 + 72 10^2 10^2 +72 (10^2)^3 ...
this is geometric series with
g = 72*10^2
r = 10^2
so
s = 72*10^2/(1.01)
= .72/.99
= 8/11
I do not understand the last one. 
thanks for the help. are you possitive they are correct?

Oh good heavens, always check anything I do!