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Math Help - Geometric Sequence

  1. #1
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    Geometric Sequence

    In a geometric sequence, the sum of first four terms is 16 times the sum of the following four term, find the common ratio?

    Sn =( a(1-r^n)) /1-r

    anyone can tell me how to start or resolve this ??

    Thank you
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  2. #2
    Super Member Quacky's Avatar
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    Re: Geometric Sequence

    Looks to me to imply:

    S_4=16(S_8-S_4). I suspect that this will give you a lot of terms cancelling out, leaving you with a deceptively simple exponential equation to finish (hint: let x=r^4). I don't know if there's a better approach.
    Last edited by Quacky; December 19th 2011 at 02:51 AM.
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  3. #3
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    Re: Geometric Sequence

    Quote Originally Posted by gilagila View Post
    In a geometric sequence, the sum of first four terms is 16 times the sum of the following four term, find the common ratio?

    Sn =( a(1-r^n)) /1-r

    anyone can tell me how to start or resolve this ??

    Thank you
    I'd use the difference of two squares (repeatedly if necessary) to clear the denominator since there is bound to be a common factor somewhere.

    If the sum of the first four terms is larger than the sum of the next four then |r| < 1. This will come in useful for checking the answer.

    S_4 = 16(S_8-S_4)


    S_4 = \dfrac{a(1-r^4)}{1-r} = \dfrac{a(1-r^2)(1+r^2)}{1-r}= a(1+r)(1+r^2)


    S_8 = \dfrac{a(1-r^8)}{1-r} = \dfrac{a(1-r^4)(1+r^4)}{1-r} = a(1+r^4)(1+r^2)(1+r)



    S_8 - S_4 = a(1+r^4)(1+r^2)(1+r) - a(1+r)(1+r^2)

    S_8 - S_4 = \underbrace{a(1+r)(1+r^2)}_{\text{ This is a common factor}}(1+r^4-1) = a(1+r)(1+r^2)r^4


    Now to put this expression back into the original expression:

    \underbrace{a(1+r)(1+r^2)}_{\text{ This is }S_4} = 16\underbrace{a(1+r)(1+r^2)r^4}_{\text{This is }S_8-S_4}


    Do some cancelling and your equation will become clear. Since you have an even power of r there are two possible answers
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  4. #4
    Super Member Quacky's Avatar
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    Re: Geometric Sequence

    Quote Originally Posted by e^(i*pi) View Post
    I'd use the difference of two squares (repeatedly if necessary) to clear the denominator since there is bound to be a common factor somewhere.
    Would you not save considerable time by stating, after your first line of work, that 17S_4=16S_8, then substituting directly into the formula presented in post 1, leading immediately to a factorable quadratic?
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  5. #5
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    Re: Geometric Sequence

    Quote Originally Posted by gilagila View Post
    In a geometric sequence, the sum of first four terms is 16 times the sum of the following four term, find the common ratio?

    Sn =( a(1-r^n)) /1-r

    anyone can tell me how to start or resolve this ??

    Thank you
    The geometric sequence is

    a,\;\;ar,\;\;ar^2,\;\;ar^3,\;\;ar^4,\;\;ar^5,\;\;a  r^6,\;\;ar^7

    T_5+T_6+T_7+T_8=r^4S_4

    S_4=16r^4S_4\Rightarrow\ r^4=\frac{1}{16}

    gives the 2 non-trivial solutions.
    Last edited by Archie Meade; December 19th 2011 at 06:08 AM.
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  6. #6
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    Re: Geometric Sequence

    Hello, gilagila!

    In a geometric sequence, the sum of first four terms
    is 16 times the sum of the following four term.
    Find the common ratio.

    We are given that:. a_1 + a_2 + a_3 + a_4 \;=\;16(a_5+a_6+a_7+a_8)

    Hence: . a + ar + ar^2 + ar^3 \;=\;16(ar^4 + ar^5 + ar^6 + ar^7)

    Divide by a\!:\;\;1 + r + r^2 + r^3 \;=\;16(r^4 + r^5 + r^6 + r^7)

    Factor: . 1 + r + r^2 + r^3 \;=\;16r^4(1+r+r^2+r^3)

    . . 16r^4(1+r+r^2+r^3) - (1+r+r^2+r^3) \;=\;0

    Factor: . (1+r+r^2+r^3)(16r^4-1) \;=\;0


    We have two equations to solve:


    r^3 + r^2 + r + 1 \:=\:0 \quad\Rightarrow\quad r^2(r+1) + (r+1) \:=\:0

    . . (r+1)(r^2+1) \:=\:0 \quad\Rightarrow\quad \boxed{r \:=\:-1}


    16r^4-1 \:=\:0 \quad\Rightarrow\quad r^4 \:=\:\tfrac{1}{16} \quad\Rightarrow\quad \boxed{r \:=\:\pm\tfrac{1}{2}}

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  7. #7
    Super Member Quacky's Avatar
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    Re: Geometric Sequence

    Quote Originally Posted by Soroban View Post
    Hello, gilagila!


    We are given that:. a_1 + a_2 + a_3 + a_4 \;=\;16(a_5+a_6+a_7+a_8)

    Hence: . a + ar + ar^2 + ar^3 \;=\;16(ar^4 + ar^5 + ar^6 + ar^7)

    Divide by a\!:\;\;1 + r + r^2 + r^3 \;=\;16(r^4 + r^5 + r^6 + r^7)

    Factor: . 1 + r + r^2 + r^3 \;=\;16r^4(1+r+r^2+r^3)

    . . 16r^4(1+r+r^2+r^3) - (1+r+r^2+r^3) \;=\;0

    Factor: . (1+r+r^2+r^3)(16r^4-1) \;=\;0


    We have two equations to solve:


    r^3 + r^2 + r + 1 \:=\:0 \quad\Rightarrow\quad r^2(r+1) + (r+1) \:=\:0

    . . (r+1)(r^2+1) \:=\:0 \quad\Rightarrow\quad \boxed{r \:=\:-1}


    16r^4-1 \:=\:0 \quad\Rightarrow\quad r^4 \:=\:\tfrac{1}{16} \quad\Rightarrow\quad \boxed{r \:=\:\pm\tfrac{1}{2}}

    With all due respect, within a reasonable time period before your reply, there were already 3 valid solutions presented which guided gilagila towards a solution, whilst allowing him to finish the work for himself. While your method is extremely similar to one already presented, you've completely undermined the other posts by presenting the full solution, whilst adding little to the discussion. I'm not ranting other than to say that I find this slightly irritating.
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  8. #8
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    Re: Geometric Sequence

    Quote Originally Posted by Quacky View Post
    With all due respect, within a reasonable time period before your reply, there were already 3 valid solutions presented which guided gilagila towards a solution, whilst allowing him to finish the work for himself. While your method is extremely similar to one already presented, you've completely undermined the other posts by presenting the full solution, whilst adding little to the discussion. I'm not ranting other than to say that I find this slightly irritating.
    Soroban is well-known for doing this at other sites...looks like he needs hugs
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  9. #9
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    Re: Geometric Sequence

    thanks for all the people helping me, a simple question but I saw various of solutions, open a new world to me.

    thank you very much
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  10. #10
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    Re: Geometric Sequence

    Quote Originally Posted by Archie Meade View Post
    The geometric sequence is

    a,\;\;ar,\;\;ar^2,\;\;ar^3,\;\;ar^4,\;\;ar^5,\;\;a  r^6,\;\;ar^7

    T_5+T_6+T_7+T_8=r^4S_4

    S_4=16r^4S_4\Rightarrow\ r^4=\frac{1}{16}

    gives the 2 non-trivial solutions.
    sorry to ask again

    T_5+T_6+T_7+T_8=r^4S_4 <--- how u get this ??
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  11. #11
    MHF Contributor Siron's Avatar
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    Re: Geometric Sequence

    T_5+T_6+T_7+T_8=ar^4+ar^5+ar^6+ar^7=r^4(a+ar+ar^2+  ar^3)=r^4S_4
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  12. #12
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    Re: Geometric Sequence

    Quote Originally Posted by Siron View Post
    T_5+T_6+T_7+T_8=ar^4+ar^5+ar^6+ar^7=r^4(a+ar+ar^2+  ar^3)=r^4S_4
    oops....never noticed that if factories that can be like that, thank you very much
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