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Math Help - Sketching graph of quadratic function

  1. #1
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    Sketching graph of quadratic function

    Diagram shows the graph of y=8-2(x-a)^2.



    find the value of a and b..
    Attached Thumbnails Attached Thumbnails Sketching graph of quadratic function-graph-y-8-2-x-2.bmp  
    Last edited by patoshik; January 31st 2010 at 05:18 AM.
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  2. #2
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    Someting wrong with the picture?

    First look for the y-intercept: Set x = 0 that gives you y = ?
    Then you wan´t to find the x intercepts. So set y = 0
    Then 2(x-2)^2 must equal 8..

    But you graph has a maximum of 8 when x = 1 seting x = 1 into the equation gives us:
    y = 8- 2(1-2)^2
    Y = 8-2(-1)^2<br />
y = 8-2 = 6   ???
    Either the graph is not entierly correct or the equation is not..
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  3. #3
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    Quote Originally Posted by Henryt999 View Post
    First look for the y-intercept: Set x = 0 that gives you y = ?
    Then you wan´t to find the x intercepts. So set y = 0
    Then 2(x-2)^2 must equal 8..

    But you graph has a maximum of 8 when x = 1 seting x = 1 into the equation gives us:
    y = 8- 2(1-2)^2
    Y = 8-2(-1)^2<br />
y = 8-2 = 6   ???
    Either the graph is not entierly correct or the equation is not..
    hello..sorry,i write a wrong question.but i have edit it .
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  4. #4
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    OK no prob

    You understood how to find b? Set x = 0
    But first you must find a.
    Do that by entering the cordinates for the vertex at (1;8)
    Set x = 1; and y = 8 into the above equation..then solve for a.
    Can you take it from here?
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  5. #5
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    can u teach me step by step?
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  6. #6
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    Sure.

    You have an equation y = 8-2(x-a)^2
    Then you have a vertex at (1;8)
    Then means when x = 1 then y = 8
    Now if you enter those values into the equation that is: 8 = 8-2(1-a)^2
    Now you have one equation and one unknown.
    Then you solve for a.
    8 = 8-2(1-a)^2
    8 = 8-2(1-a)^2. notice that on the left side equals 8. then the right side must equal eight.
    The only way the right side can equal 8 is when the expression -2(1-a)^2 = 0
    that gives you 8 = 8.
    For what values of a does the exspression equal 0???
    There is only one value for a that is possible....
    Once you done that you want to find b.
    To find b. Enter x = 0 into your equation. like this:
    y = 8-2(0-a)^2 by now you know what a is so whatever value comes out that is your b. also called the y intercept.
    Can you take it from here?
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  7. #7
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    thanks a lot
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