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Math Help - Did I prove this correctly?

  1. #1
    Super Member sakonpure6's Avatar
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    Did I prove this correctly?

    Hi, there is this prove question that I did, but I am not sure If I did it correctly. I mean, am I aloud to manipulate both sides of the equation at the same time, and is it considered correct to end up with what I did?? Am I supposed to deal with this like an identity?

    Did I prove this correctly?-untitled.pngDid I prove this correctly?-20140127_093704.jpg
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  2. #2
    MHF Contributor ebaines's Avatar
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    Re: Did I prove this correctly?

    It's hard to read the attachment, but it looks like the first line is

     h^2 + k^2 = 23 hk

    Please correct me if I misread that. Then on the second line you have written:

     \log(\frac {h+k} 5 ) = \frac 1 2 ( \log h + \log k)

    Not sure how you got that, but in general it's not true. Using the identity:  \log( \frac a b ) \ \log a - \log b in general:

     \log(\frac {h+k} 5 ) = \log(h+k) - \log 5

    It would help if can tell us what it is you are trying do with this problem.
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  3. #3
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    Re: Did I prove this correctly?

    Quote Originally Posted by sakonpure6 View Post
    Hi, there is this prove question that I did, but I am not sure If I did it correctly. I mean, am I aloud to manipulate both sides of the equation at the same time, and is it considered correct to end up with what I did?? Am I supposed to deal with this like an identity?

    Click image for larger version. 

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ID:	30089Click image for larger version. 

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    you worked the problem backwards. You need to start with h^2 + k^2 = 23 h k as in

    h^2 + k^2 = 23 h k \Rightarrow

    (h+k)^2=25 h k \Rightarrow

    \left(\frac{h+k}{5}\right)^2=hk \Rightarrow

    2\log\left(\frac{h+k}{5}\right)=\log(h)+\log(k) \Rightarrow

    \log\left(\frac{h+k}{5}\right)=\frac{\log(h)+\log(  k)}{2}
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  4. #4
    Super Member sakonpure6's Avatar
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    Re: Did I prove this correctly?

    If I worked backwards, is that still considered correct?
    @ebaines I attached the question
    Last edited by sakonpure6; January 27th 2014 at 07:15 AM.
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    Re: Did I prove this correctly?

    Quote Originally Posted by sakonpure6 View Post
    If I worked backwards, is that still considered correct?
    @ebaines I attached the question
    not really but if you can work it backwards it's usually a trivial matter to just write it down in reverse order.
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  6. #6
    MHF Contributor ebaines's Avatar
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    Re: Did I prove this correctly?

    Quote Originally Posted by sakonpure6 View Post
    If I worked backwards, is that still considered correct?@ebaines I attached the question
    You didn't really do it backwards - that would have meant starting with  \log ( \frac {h+k}5 ) = \frac 1 2 (( \log h + \log k) and working from that to arrive at  h^2 + k^2 = 23 hk. Your approach was a bit different - you started working backwards but mid-way threw in the initial given equation to ultimately arrive at 1 = 1, and hence conclude that the original statement must therefore be true. That works, but to avoid confusion like I had when I first saw your work you should notate your work to help the reader folllow your logic. The problem is that if you simply write a series of equations the reader assumes that each equation comes naturally from the one before it. But in your work it doesn't flow like that, so it's confusing. It's best to start with the given fact that  h^2 + k^2 = 23hk and from that proceed through a series of steps where each derives from the previous, to ultimately arrive at the desired expression.
    Last edited by ebaines; January 27th 2014 at 11:00 AM.
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  7. #7
    Super Member sakonpure6's Avatar
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    Re: Did I prove this correctly?

    Alright , thank you guys!
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