math

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

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asked by Jon
  1. 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
    a1 = -3
    aN+1 = 3N - n

    then:
    a1 = -3
    a2 = 3a1 - 2 = -11
    a3 = 3a2 - 3 = -36
    a4 = 3a3 - 4 = -112
    a5 = 3a4 - 5 = -341 which is choice D

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    posted by Reiny
  2. find a1 for given geometric series.

    Sn= 405, r=3, n=4

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    posted by Isabel

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