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Math Help - quadratic expressions help

  1. #1
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    quadratic expressions help

    quadratic expressions help-pls.jpgI just cant make sense of this, ive tried doing research on quadratic expressions and what not but i really dont know what im doing here, any help would be appreciated. Thanks.
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  2. #2
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    Re: quadratic expressions help

    They have already defined the relevant variables, namely

    $d = quantity\ sold\ (in\ thousands\ of\ pounds);$

    $p = sales\ price\ per\ unit;$

    $r = revenue\ (in\ thousands\ of\ pounds);$

    $c = cost\ of\ production\ (in\ thousands\ of\ pounds);\ and$

    $n = profit\ (in\ thousands\ of\ pounds).$

    Nothing hard about that. It has already been given to you.

    And they have given you a number of relationships, namely

    $n = r - c;$

    $r = d * p;$

    $p = 16 - 2d;\ and$

    $c= 8 + 3d.$

    Again that is given to you. All this problem is asking you to do is first to find r in terms of d, which involves working out

    $r = p * d = (16 - 2d) * d = what?$ Notice that you now have r equal to an expression in d, which just happens to be quadratic.

    Then it wants you to find n in terms of d, which involves working out

    $n = r - c = r - (8 + 3 * d) = what\ in\ terms\ of\ d?$

    Again it just happens that the final expression in d is a quadratic.

    You probably would have got this if the word "quadratic" had not been there. There is no necessity for sales, cost, revenue, or profit to be quadratics. It just works out that IN THIS CASE revenue and profit are quadratic.

    Edit: By the way, this type of question should be put in the Business Math forum in the future. You will get a quicker answer.
    Last edited by JeffM; August 6th 2014 at 11:51 AM.
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  3. #3
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    Re: quadratic expressions help

    im still a little confused on how to figure out D ;/
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  4. #4
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    Re: quadratic expressions help

    so does this mean that D is equal to 8, still rather confused on this
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  5. #5
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    Re: quadratic expressions help

    You are not asked to find d. You are asked to get all the variables expressed in terms of d.
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  6. #6
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    Re: quadratic expressions help

    well, you certainly have me confused!

    There is NO "D" in the problem. If you mean "d", there is no part of the problem that asks you to calculate d.
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  7. #7
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    Re: quadratic expressions help

    theres a reason im asking for help ;-; but i think im starting to understand what im supposed to do.
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  8. #8
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    Re: quadratic expressions help

    Quote Originally Posted by mybeans View Post
    theres a reason im asking for help ;-; but i think im starting to understand what im supposed to do.
    In post #2, I explained what needs to be done.

    Way back, when you were studying beginning algebra, you were undoubtedly asked to express multi-variable equations in terms of a given variable.

    Example: Express y in terms of x given $x^3 + 3x^2y + 3xy^2 + y^3 = 27y^3.$

    $x^3 + 3x^2y + 3xy^2 + y^3 = 27y^3\implies (x + y)^3 = (3y)^3 \implies x + y = 3y \implies 2y = x \implies y = \dfrac{x}{2}.$

    This is a similar problem.
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