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Math Help - Equation of parabola!

  1. #1
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    Equation of parabola!

    How the hell do you derive an equation from a graph?

    For example this inverted parabola has x-intercepts at 0 and 2, but does that really mean that the equation is -x(x-2)?

    Well it cant be, because the vertex is at (1,2), and -1(1-2)=1, not 2. How do you derive equations from graphs??
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  2. #2
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    Quote Originally Posted by olivia59 View Post
    How do you derive an equation from a graph?

    For example this inverted parabola has x-intercepts at 0 and 2, but does that really mean that the equation is -x(x-2)?

    Well it cant be, because the vertex is at (1,2), and -1(1-2)=1, not 2. How do you derive equations from graphs??
    x-intercepts at x = 0 and x = 2

    y = kx(x-2) , where k is a constant TBD ....

    vertex at (1,2) ...

    2 = k(1)(1-2)

    2 = -k

    k = -2

    equation is y = -2x(x-2)


    btw ... watch your language.
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  3. #3
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    Thanks, sorry.
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  4. #4
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    Quote Originally Posted by olivia59 View Post
    How the hell do you derive an equation from a graph?

    For example this inverted parabola has x-intercepts at 0 and 2, but does that really mean that the equation is -x(x-2)?

    Well it cant be, because the vertex is at (1,2), and -1(1-2)=1, not 2. How do you derive equations from graphs??
    Also "watch your language" in writing "the equation is -x(x-2)". That's not even an equation!

    You meant "does that mean the equation is y= -x(x-2)?"

    And the answer is "no". It means the equation is y= ax(x-2) where a is some negative number. You can use the fact that when x= 1, y= 2 to determine a: 2= a(1)(1- 2)= -a.
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