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Math Help - To use parentheses or not to

  1. #1
    Newbie
    Joined
    May 2009
    Posts
    2

    To use parentheses or not to

    That is the question

    First post for me, so hi everyone.

    I'm uncertain if this is the correct subforum for this question, I'm from Sweden and having a bit of a problem with what's Algebra and what is not since we do not use the term.

    But anyhow, here's my problem.

    I'm doing some derivative problems (think it's derivative in english at least ) and I ran into a problem which isn't even related to the derivative but I'll just punch out the whole thing:

    y=x^3-3x^2

    Which then comes to

    y'=3x^2-6x
    0=3x^2-6x
    0=3x(x-2)
    x1=0 x2=2

    And now I wanna see if it's a positive or negative tangent prior to the first bend.

    And here comes my problem, I am uncertain if I should put the number in parentheses or not, if I do it looks like this:

    y'(-1)=3*(-1)^2-6*(-1)=9

    If not it's like this:

    y'(-1)=3*-1^2-6*-1=3

    Which is the correct one

    Thanks in advance
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  2. #2
    MHF Contributor
    Joined
    Mar 2007
    Posts
    1,240

    Talking

    If you mean that you're evaluating at x = -1, then definitely use the parentheses! Otherwise, for instance, -1^2 = -(1^2) = -1, which is not what you mean!
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  3. #3
    Member
    Joined
    Apr 2009
    Posts
    166
    You're replacing the whole x with -1. So x^2 becomes (-1)^2.

    y'(-1)=3*(-1)^2-6*(-1)=9
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  4. #4
    Newbie
    Joined
    May 2009
    Posts
    2
    Wow!

    Thanks a bunch for the quick answers!

    I was pretty sure I had to use the parentheses but it's one of those things that I know I've asked my teacher about and he said something like "XXXX use parentheses" and I couldn't remember if the XXXX was always or never

    Thanks again guys
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