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Math Help - Logs two equtions

  1. #1
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    Logs two equtions

    \log_3(x)+\log_9(y)=1
    \log_x(3)+\log_y(9)=-\frac{1}{2}

    How can i solve this?

    Thank you!
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  2. #2
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    Quote Originally Posted by mathwhat View Post
    \log_3(x)+\log_9(y)=1
    \log_x(3)+\log_y(9)=-\frac{1}{2}

    How can i solve this?

    Thank you!
    Are you asking to solve these equations simultaneously for x and y?


    I'd try converting everything to the natural logarithm using the change of base rule \log_b{x} = \frac{\log_k{x}}{\log_k{b}}.


    So Equation 1:

    \log_3{x}+\log_9{y}=1

    \frac{\ln{x}}{\ln{3}} + \frac{\ln{y}}{\ln{9}} = 1

    \frac{\ln{x}}{\ln{3}} + \frac{\ln{y}}{2\ln{3}} = 1

    \frac{2\ln{x} + \ln{y}}{2\ln{3}} = 1

    2\ln{x} + \ln{y} = 2\ln{3}

    \ln{y} = 2\ln{3} - 2\ln{x}.


    Equation 2:

    \log_x{3}+\log_y{9}=-\frac{1}{2}

    \frac{\ln{3}}{\ln{x}} + \frac{\ln{9}}{\ln{y}} = -\frac{1}{2}

    \frac{\ln{3}}{\ln{x}} + \frac{2\ln{3}}{\ln{y}} = -\frac{1}{2}

    \frac{\ln{3}\ln{y} + 2\ln{3}\ln{x}}{\ln{x}\ln{y}} = -\frac{1}{2}

    2\ln{3}\ln{y} + 4\ln{3}\ln{x} = -\ln{x}\ln{y}

    2\ln{3}\ln{y} - \ln{x}\ln{y} = -4\ln{3}\ln{x}

    \ln{y}(2\ln{3} - \ln{x}) = -4\ln{3}\ln{x}

    \ln{y} = \frac{-4\ln{3}\ln{x}}{2\ln{3} - \ln{x}}.


    Now you can set the equations equal to each other and solve.
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  3. #3
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    Thank you but i don't familiar with \ln...
    I am only know the \log thing...
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  4. #4
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    Quote Originally Posted by mathwhat View Post
    Thank you but i don't familiar with \ln...
    I am only know the \log thing...
    \ln is just shorthand for the logarithm with base e.

    I.e. \ln{x} = \log_e{x}.


    If you want you can use the base 10... It doesn't make any difference. The point of using the change of base rule is to get all the logarithms to HAVE THE SAME BASE.
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  5. #5
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    Thank you very much for the useful information!!!
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  6. #6
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    Red face

    hi this is my answer i dont know its right cuz im not good at math...but hopefully it's right from my mention
    just anohter thing i dont know to much latex language so ....

    <br />
log_3{x}+ log_9{y} = 1<br />
    <br />
log_3{x}+log_3^2{y}=1   <br />
    here is log base 3 SQUARED of y but cant do it with latex
    <br />
\log_3{9}=\log_3{3^2}=2\log_3{3}=2
    its another role of changing base now example
    \log_9{3}=\log_3^2{3}=\frac{1}{2}\log_3{3}=\frac{1  }{2}

    <br />
log_3{x}+\frac{1}{2}log_3{y}=log_3{3}<br />

    we know that log base 3 of number 3 is 1 so we can write 1 as log base 3 of 3


    now we remove logs and we got

    <br /> <br />
x+\frac{1}{2}y=1                                 <br /> <br />
    hope im right and you solve other equation
    Last edited by icefirez; April 29th 2010 at 07:22 AM.
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