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Math Help - Logs help

  1. #1
    Junior Member
    Joined
    Jan 2008
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    53

    Logs help

    How do i figure these out?:

    The expression log(base 2)*(x-4) is undefined for all values of x such that
    (1) x>1 (2) x<or equal to 4 (3) x>0 (4) x<or equal to 0

    The expression log(base 3)*(8-x) is defined for all values of x such that
    (1) x>8 (2)x<8 (3)x>or equal to 8 (4)x<or equal to 8

    All i need to know is how to do them, i have the answers.
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  2. #2
    Newbie
    Joined
    Jan 2009
    Posts
    14
    hello,
    you just need to know the domain of a logarithmic function.
    which is: base must be positive,and different from 1,
    while the result of the logarithm must be greater than zero
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  3. #3
    MHF Contributor
    Joined
    Mar 2007
    Posts
    1,240

    Talking

    Quote Originally Posted by eh501 View Post
    How do i figure these out?
    Logarithms are defined only for positive inputs. In particular, they are undefined for any input that is zero or less.

    So, to "figure these out", take the input expression and set it "less than or equal to zero" and solve to find where the log is not defined, or set it "greater than zero" and solve to find where the log is defined.

    Have fun!
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