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Thread: Finding a quadratic function from given information

  1. October 16th 2009, 02:02 AM #1
    Tessarina
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    Exclamation Finding a quadratic function from given information

    Using this information, can you find the quadratic equation.

    Y intercept- (o,-8)

    X intercepts- (1,0) and (4,0)



    Please try, because i jut do not understand ANY of it
    Last edited by mr fantastic; October 17th 2009 at 09:42 PM.
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  2. October 16th 2009, 02:31 AM #2
    Grandad
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    Hello Tessarina
    Quote Originally Posted by Tessarina View Post
    Using this information, can you find the quadratic equation.

    Y intercept- (o,-8)

    X intercepts- (1,0) and (4,0)



    Please try, because i jut do not understand ANY of it
    Suppose the equation is of the form y = ax^2+bx+c. Then plug in each of the pairs of values of x and y in turn:

    (0,-8):-8 = a0^2+b0+c
    \Rightarrow c = -8
    (1,0):0 = a+b+c
    \Rightarrow a+b=8 (1)
    (4,0): 0 = 16a+4b+c
    \Rightarrow 16a+4b=8
    \Rightarrow 4a+b=2 (2)
    Now solve the simultaneous equations (1) and (2) to find a and b, and plug the answers back into the original equation to get
    y=-2x^2+10x-8
    Can you fill in the working?

    Grandad
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