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Math Help - quadrativ question: easy to answer

  1. #1
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    quadrativ question: easy to answer

    I'm getting a little confused with the quadrativ formula, i used to know it, but its been a while!

    i'm getting -b+/- square root (b-4ac) / 2* c

    the part im getting confused on is the 2 * c, what if my original equation was x^2 - 10x - 3 (where -3 would be c)

    do i add the minus sign into the equation which would give me 2 * -3 = -6 and then divide the whole equation by -6 or do i forget the minus sign and change it to positive 6?
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  2. #2
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    The quadratic formula is \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}

    2a on the bottom NOT 2c.
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  3. #3
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    Quadratic Formula

    Hello
    Quote Originally Posted by entrepreneurforum.co.uk View Post
    I'm getting a little confused with the quadrativ formula, i used to know it, but its been a while!

    i'm getting -b+/- square root (b-4ac) / 2* c

    the part im getting confused on is the 2 * c, what if my original equation was x^2 - 10x - 3 (where -3 would be c)

    do i add the minus sign into the equation which would give me 2 * -3 = -6 and then divide the whole equation by -6 or do i forget the minus sign and change it to positive 6?
    You're asking two things here: the first is about the formula itself, and the second is how to handle minus signs. So:

    1 The formula you want is

    x=\frac{-b \pm \sqrt{b^2-4ac}}{2a}

    2 Handling minus signs. Remember the rules for multiplying and dividing positive ( +^{ve}) and negative ( -^{ve}) numbers:

    +^{ve}\times +^{ve}=+^{ve}

    +^{ve}\times -^{ve}=-^{ve}

    -^{ve}\times +^{ve}=-^{ve}

    -^{ve}\times -^{ve}=+^{ve}

    So, do you just forget the minus signs and change them to positive? Definitely not!

    Taking your example:

    x^2 - 10x - 3 gives a=1, b=-10 and c=-3

    Plugging these values into the formula, you get:

    x=\frac{-(-10) \pm \sqrt{(-10)^2-4(1)(-3)}}{(2)(1)}

    (Where I've written two numbers in brackets next to one another - like (1)(-3), there's a \times sign in between; so that means (1) \times (-3).)

    So x = \frac{10 \pm \sqrt{100+12}}{2}

    Can you see where all the minus signs have gone, using the rules above?

    I hope that helps to clear one or two problems up.

    Grandad
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