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Math Help - Quadratic Equations

  1. #1
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    Quadratic Equations

    Hello Everybody

    Im not in high School, but I am studying for my GMAT and need some help with some stuff that I havnt done in years.

    First

    What are the formulas for finding the intercepts of a quadratic equation.

    I think negetive b plus minus the square root of 4ac divided by 2a is this correct? which intercept does this give you? Whats the formula for finding the other intercept?

    How do you solve a problem where you are given the intercepts and asked to choose which equation?

    For example:

    What equation has te intercepts of (-1,-5)

    No graphing calculators allowed or else this would be easy.
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  2. #2
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    Quote Originally Posted by erinwithane View Post
    Hello Everybody

    Im not in high School, but I am studying for my GMAT and need some help with some stuff that I havnt done in years.

    First

    What are the formulas for finding the intercepts of a quadratic equation.

    I think negetive b plus minus the square root of 4ac divided by 2a is this correct? which intercept does this give you? Whats the formula for finding the other intercept?

    How do you solve a problem where you are given the intercepts and asked to choose which equation?

    For example:

    What equation has te intercepts of (-1,-5) Mr F says: If you mean the equation of a graph that passes through the point (-1, -5) then there are an infinite number of possible answers. eg. any line of the form y + 1 = m(x + 5) where m is the (non-zero) gradient of the line.

    No graphing calculators allowed or else this would be easy.
    The solution to ax^2 + bx + c = 0 is x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}.

    I strongly advise that you get a hold of a textbook that covers the material in the GMAT.
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  3. #3
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    isnt there another equation to find Y? I specifically remember there being one, but I cant remember what it is. i had an Algebra Teacher is 8th grade tell me what it was but I cant remember it, and I cant find it online anywhere.

    I have a GMAT Study guide but it doesnt do a great job of explaining this stuff.
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  4. #4
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    Quote Originally Posted by erinwithane View Post
    isnt there another equation to find Y? I specifically remember there being one, but I cant remember what it is. i had an Algebra Teacher is 8th grade tell me what it was but I cant remember it, and I cant find it online anywhere.
    [snip]
    Perhpas if you posted the exact question a more specific answer could be given.

    Quote Originally Posted by erinwithane View Post
    [snip]
    I have a GMAT Study guide but it doesnt do a great job of explaining this stuff.
    Then get a hold of another book. Library might be a good place to start looking.
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  5. #5
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    The specific question is what is the formula for finding the Y intercept.
    Last edited by CaptainBlack; November 2nd 2008 at 01:09 AM.
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  6. #6
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    Quote Originally Posted by erinwithane View Post
    The specific question is what is the formula for finding the Y intercept.

    If you dont know, maybe somebody else does.
    I'm not a mind reader. If you cannot ask a clear question you will not get a clear answer.

    To find the y-intercept you substitute x = 0 and solve for y. Do find the x-intercept you substitute y = 0 and solve for x.

    Take my advice and get a textbook.


    By the way, if you spent a small amount of time looking through these forums you would quickly realise why I find your remark
    Last edited by ThePerfectHacker; November 2nd 2008 at 01:51 PM.
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  7. #7
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    Mr F stated


    img.top {vertical-align:15%;} x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}<br />
" alt="

    x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}<br />
" />

    This is the formula for X.

    My question is what is the formula for Y?
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  8. #8
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    Quote Originally Posted by erinwithane View Post
    Mr F stated


    img.top {vertical-align:15%;} x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}<br />
" alt="

    x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}<br />
" />

    This is the formula for X.

    My question is what is the formula for Y?
    If y = ax^2 + bx + c then the y-intercept is (0, c). However, if the parabola is not written in standard form then this formula is useless (as are most formulas if you don't understand where they come from).

    In general to get the y-intercept you have to substitute x = 0 and solve for y.

    Next time try being clearer in what you want to know. It will save a lot of time.
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