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Math Help - Forms of Quadratic Functions HELP!

  1. #1
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    Forms of Quadratic Functions HELP!

    Hi everyone!
    Some of the problems below are answered but I need someone to check if I'm correct but some problems are also empty and those are the ones I am having trouble with. Please help! Thank you so much.

    ---

    f(x) = x^2-8x-9

    a) Give f(x) in factored form.
    Answer: f(x) = (x-9)(x+1)

    b) Give f(x) in vertex form.
    Answer: f(x) = (x-4)^2-25

    c) Give the vertex of f(x) and its axis of symmetry.
    Answer: Not sure how to get this!

    d) Does f(x) have an absolute minimum OR an absolute maximum? What is its minimum or maximum value?
    Answer: Not sure how to get this!

    e) Give the interval on which f(x) is decreasing.
    Answer: Not sure how to get this!

    f) What are the zeros of f(x) ? (its x-intercepts)
    Answer: x-intercepts => (9,0) and (-1,0)

    g) What is the y-intercept of f(x) ?
    Answer: y-intercept => (0,-9)

    ---

    Please help me on the red parts =\ THANK YOU!
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  2. #2
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    Quote Originally Posted by RedSpades View Post
    c) Give the vertex of f(x) and its axis of symmetry.
    Answer: Not sure how to get this!
    For a parabola,
    y=ax^2+bx+c,a\not = 0.
    The axis of symettry is always.
    -\frac{b}{2a}.
    Use this to answer your question.
    d) Does f(x) have an absolute minimum OR an absolute maximum? What is its minimum or maximum value?
    Look at the coefficeint with the square.
    It is positive.
    So it is a parabola opening up, hence an absolute minimum.
    e) Give the interval on which f(x) is decreasing.
    Answer: Not sure how to get this!
    Since it is a parabola opening up everything to the vertex is the decreasing interval and everything after is an increasing interval. Use part (c) to find the vertex.
    -----
    I question. Do you call yourself "Redspades" because you use the Black Bicycle Deck?
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