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Math Help - quadratic expression

  1. #1
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    quadratic expression

    The diagram shows part of the graph of the function: y=ax^2+bx+c.

    (seeing as I cannot draw the diagram here, it is basically an upside down parabola, with the vertex on the x-axis, and the whole parabola is in the fourth quadrant)

    State whether each of the following expressions is positive, negative or zero.

    a) a negative
    b) c negative
    c) b^2-4ac zero
    d) b ?

    I cannot figure out how to find whether constant b is positive, negative or zero. Can someone help me?
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  2. #2
    Senior Member Stroodle's Avatar
    Joined
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    I guess one way to do it is:

    y=a\left (x^2-\frac{b}{a}x\right )+c

    y=a\left ( x-\frac{b}{2a}\right )^2 -\frac{b^2}{4a}+c

    So the graph would be translated \frac{b}{2a} units to the right. But in your graph the vertex is to the left, and a is negative. Therefore b must be positive.
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