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Math Help - find the specified value for r and n

  1. #1
    rcs
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    find the specified value for r and n

    can anybody here help me on this problem.... i've been trying to solve them with the use of the formula of geometric sequence and geometric series. but i could not make it to the answer.

    Given: 1. S sub 5 = 31/4 ; r = 1/2 ; what is first term?

    2. S sub n = 94.5 ; r = 1/2 ; a sub n = 3/2 ; what is n?

    Thank you so much
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  2. #2
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    1. \displaystyle \frac{31}{4} = \frac{a\left[1-\left(\frac{1}{2}\right)^5\right]}{1-\frac{1}{2}}. Simplify and solve for \displaystyle a.


    2. \displaystyle 94.5 = \frac{\frac{3}{2}\left[1 - \left(\frac{1}{2}\right)^n\right]}{1 - \frac{1}{2}}. Simplify and solve for \displaystyle n.
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  3. #3
    rcs
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    thank you Prove it. it is ok to put 3/2 which is the n value in the place of a sub 1 or first term?
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