Algebra

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Find the sum of the first four terms of the geometric sequence with a = -3 and r = 3.

I have NO idea how to attempt to solve this. I do not want an answer just instuctions. Thanks

  • Algebra -

    The terms in the sequence are presumably
    a*r^n.
    I rceommend that you read
    http://en.wikipedia.org/wiki/Geometric_progression

    The first four terms would be (starting with n=0),
    a, a r, ar^2 and a r^3.

    The sum is a (1 + r + r^2 + r^3)
    -3 * (1 + 3 + 9 + 27)
    = -120

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