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Math Help - Graphing log Function

  1. #1
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    Graphing log Function

    Graph
    y=log base 3 (x+7)
    It asks to graph, but i am just having problem solving for points(Y)

    I know that you can also graph this function in exponential form
    3^y=x+7

    however, i tried to substitute in for x in the log function, but getting wrong answers
    x=-7
    y=log base 3(-7+7) = 1

    expo
    x=-6
    3^y=-6+7
    3^y=1
    y=1

    I am substituting different numbers for x but getting same answer, in fact the -6 is correct not the -7.

    How do i find y in a using the log function?

    Thank You
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  2. #2
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    Quote Originally Posted by hovermet View Post
    Graph
    y=log base 3 (x+7)
    It asks to graph, but i am just having problem solving for points(Y)

    I know that you can also graph this function in exponential form
    3^y=x+7

    however, i tried to substitute in for x in the log function, but getting wrong answers
    x=-7
    y=log base 3(-7+7) = 1 <<<<<< log(0) is not defined!

    ...
    If you don't want to use a calculator you should get powers of 3 as argument of the log function:

    y = \log_3(x-7)
    Therefore:

    y = \log_3(8-7)~\implies~y = 0
    y = \log_3(10-7)~\implies~y = 1
    y = \log_3(16-7)~\implies~y = 2
    y = \log_3\left(\frac{22}3-7\right)~\implies~y = -1

    If you are allowed to use a calculator use the base-change-formula to get the y-values for any x > 7:

    y = \log_3(x)~\implies~y=\dfrac{\ln(x)}{\ln(3)}
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