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Math Help - Right answer but...not quite ? (Quadratic)

  1. #1
    Newbie Monocerotis's Avatar
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    Right answer but...not quite ? (Quadratic)

    So here is the question and the way I worked through it

    The answer is 55.125 cm^2

    I just feel as thought my method used to arrive at the answer is either wrong, or a misinterpretation of steps in order
    .
    It's really been bothering me, like there is an approach to the question, which is the correct procedure, and I haven't implemented it.

    Take a look ladies & gents.

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  2. #2
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    edit: I read the question again.

    here is how I would do it:

    you know that A = 1/2(21-h)*h = 10.5h - 0.5h^2

    then dA/dh = 10.5 - h

    when dA/dh = 0, h = 10.5

    when h = 10.5, b = 10.5, so A = 55.125 cm^2
    Last edited by harbottle; November 1st 2009 at 10:52 PM.
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  3. #3
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    mr fantastic's Avatar
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    Quote Originally Posted by harbottle View Post
    edit: read the question again.

    here is how I would do it:

    you know that A = 1/2(21-h)*h = 10.5h - 0.5h^2

    then dA/dh = 10.5 - h

    when dA/dh = 0, h = 10.5

    when h = 10.5, b = 10.5
    This is a good solution and leads to the same answer as the OP got. However, since the OP posted in the Pre-algebra and Algebra subforum it's likely that s/he hasn't studied calculus and therefore may not understand it. As a general rule, an expectation of replies to questions posted in this subforum would be that non-calculus methods be used.

    @OP: You have made heavy weather of finding the zeros of the area function. Since the original expression was factorised, it ought to be evident that the zeroes are h = 21 and h = 0 using the null factor law (you might know this idea under a different name). The maximum turning point will therefore lie halfway between - at x = 10.5 etc.

    The question appears to be dealt with satisfactorily so for various reasons I'm closing this thread.
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