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Math Help - Geometric and Arithmetic Sequence

  1. #1
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    Geometric and Arithmetic Sequence

    Hi im a bit confused about these questions here. I do not know where to start, but much help is appreciated.

    Question 1

    In an arithmetic sequence t4 + t5 + t6 = 300 and t15 + t16 + t17 = 201
    Find t18

    Question 2

    In a geometric sequence t1 + t2 + t3 = 21 and t4 + t5 + t6 = 168
    Find the squence

    Thanks in advance =]
    Last edited by ferken; January 8th 2009 at 06:52 PM.
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  2. #2
    MHF Contributor red_dog's Avatar
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    In an arithmetic sequence t_n=t_1+(n-1)r
    In a geometric sequence t_n=t_1r^{n-1}

    Write the equations in terms of t_1 and  r, then solve the systems.
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  3. #3
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    Another way for the first one.

    The n-th term of an arithmetic sequence is given by t_n = t_m + (n-m)d where n > m and d is common difference.

    You are given the sum of consecutive odd-numbered terms. The arithmetic mean of these terms is the middle term. For instance:
    \frac{t_4+t_5+t_6}{3} = t_5

    Note that this applies only for the arithmetic sequence.
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