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Math Help - [SOLVED] geometric series question

  1. #1
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    [SOLVED] geometric series question

    if x-1, x+3 and x+12 are consecutive terms in a geometric sequence what is the ratio.

    I was told you put t3 over t2, and also t2 over t1 and cross multiply, which gives you a quadratic, but when I do it my (a) value is 0 so it would be

    5 +/-5 over 0
    ?

    then you sub that into the original equation? how do you sub x into tn=ar^n-1
    or sn=a(r^n-1over r-1)?

    thanks.
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  2. #2
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    Quote Originally Posted by brentwoodbc View Post
    if x-1, x+3 and x+12 are consecutive terms in a geometric sequence what is the ratio.

    I was told you put t3 over t2, and also t2 over t1 and cross multiply, which gives you a quadratic, but when I do it my (a) value is 0 so it would be

    5 +/-5 over 0
    How? What were your steps? (We can't find the errors until you show your work.)

    By nature of geometric sequences, yes, you need to set up the ratios. And since the ratio is "common", you get:

    . . . . . \frac{x\, +\, 12}{x\, +\, 3}\, =\, \frac{x\, +\, 3}{x\, -\, 1}

    Then you need to solve the rational equation.

    . . . . . (x\, -\, 1)(x\, +\, 12)\, =\, (x\, +\, 3)(x\, +\, 3)

    . . . . . x^2\, +\, 11x\, -\, 12\, =\, x^2\, +\, 6x\, +\, 9

    . . . . . 11x\, -\, 12\, =\, 6x\, +\, 9

    I'm not seeing how you arrived at division by zero...? Please reply showing your steps in solving the linear equation.

    Thank you!
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  3. #3
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    Quote Originally Posted by stapel View Post
    How? What were your steps? (We can't find the errors until you show your work.)

    By nature of geometric sequences, yes, you need to set up the ratios. And since the ratio is "common", you get:

    . . . . . \frac{x\, +\, 12}{x\, +\, 3}\, =\, \frac{x\, +\, 3}{x\, -\, 1}

    Then you need to solve the rational equation.

    . . . . . (x\, -\, 1)(x\, +\, 12)\, =\, (x\, +\, 3)(x\, +\, 3)

    . . . . . x^2\, +\, 11x\, -\, 12\, =\, x^2\, +\, 6x\, +\, 9

    . . . . . 11x\, -\, 12\, =\, 6x\, +\, 9

    I'm not seeing how you arrived at division by zero...? Please reply showing your steps in solving the linear equation.

    Thank you!
    cross multiply and get x^2+6x+p=x^2+11x-12
    which is
    0x^2-5x+21

    with quadratic equation thats

    5 +/- sqrt5 over 2x0

    =
    5 {+/-} 5 over 0

    I dont know why my teacher said to use the quadratic equation because you can just solve for x, but once you have x how do you find r?
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  4. #4
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    Quote Originally Posted by stapel View Post
    How? What were your steps? (We can't find the errors until you show your work.)

    By nature of geometric sequences, yes, you need to set up the ratios. And since the ratio is "common", you get:

    . . . . . \frac{x\, +\, 12}{x\, +\, 3}\, =\, \frac{x\, +\, 3}{x\, -\, 1}

    Then you need to solve the rational equation.

    . . . . . (x\, -\, 1)(x\, +\, 12)\, =\, (x\, +\, 3)(x\, +\, 3)

    . . . . . x^2\, +\, 11x\, -\, 12\, =\, x^2\, +\, 6x\, +\, 9

    . . . . . 11x\, -\, 12\, =\, 6x\, +\, 9

    I'm not seeing how you arrived at division by zero...? Please reply showing your steps in solving the linear equation.

    Thank you!


    I dont know why he said to use the quadratic formula because you dont need it, and it doesnt work.

    so I sub the 4.2 into the x and I get the answer of 2.25.
    Last edited by brentwoodbc; April 5th 2009 at 05:43 PM.
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