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Math Help - Quadratic function and exponential function question!

  1. #1
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    Unhappy Quadratic function and exponential function question!

    hi, it's me again. Thanks so much for the help last time.

    i have this table of values:
    x | y
    1 | 20
    3 | 320

    i have to make a linear, quadratic, and exponential function from these values.
    i used y-y1=m(x-x1) for the linear and came up with f(x)= 150x-130

    am i supposed to use the quadratic formula to find the quadratic function? i tried but I ended up being unable to find an answer (undefined?). can someone please walk me through this? maybe I just need the values for a, b, and c.

    as for the exponential function, i am completely clueless...

    as always, i am not allowed to use a calculator for these problems, so i hope someone can guide me through the steps.
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  2. #2
    Eater of Worlds
    galactus's Avatar
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    Chaneysville, PA
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    Do you have another set of data points for the quadratic?.

    As it is, you have

    a+b+c=20

    9a+3b+c=320

    We need one more because we have three variables but only two equations.

    Unless, we can assume c=0 and have y=ax^{2}+bx

    Then we have

    a+b=20

    9a+3b=320

    Then we solve the system and get

    a=\frac{130}{3}, \;\ b=\frac{-70}{3}

    For the exponent, we can use y=a\cdot{b^{t}}

    20=a\cdot{b^{1}}

    320=a\cdot{b^{3}}

    Solve for a and b.
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  3. #3
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    Joined
    Mar 2008
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    thank you!!!

    a problem with the quadratic one... how do i figure out how f(x)=170?
    if i do 170= 130x^2 + -20/3x, what are the next steps?

    for the exponential one... i came up with a=88.657... probably not right, cuz i don't think it makes sense. help please?
    also, how do i find f(x+3)?
    sorry for all these questions.
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