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Math Help - Domain of a function?!

  1. #1
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    Domain of a function?!

    f(x) = 8/x-9 and g(x) = 14/(x+3)

    all i know is that x cannot equal 9 and -3.

    what is the domain of f+g and f/g in interval notation??
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  2. #2
    MHF Contributor

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    "f+ g" is defined by "f+ g(x)= f(x)+ g(x)". In order that f+ g (x) exist, f(x) and g(x) must both exist (even if adding algebraically appears to eliminate a "division by 0").

    That means that the domain of f+ g is the intersection of the domains of f and g. The domain of f(x)= 8/(x- 9) (which is NOT what you wrote but must be what you meant since the domain of 8/x- 9 is "all real numbers except 0") is "all real numbers except 9" and the domain of g(x)= 14/(x+ 3) is "all real numbers except -3". The domain of f+ g is "all real numbers except 9 and -3". In interval notation, that would be (-\infty, -3)\cup(-3, 9)\cup(9, \infty).

    For f/g, both f and g must exist and g(x) must not be 0. Since 14/(x+ 3) is never 0 (a fraction is equal to 0 if and only if its numerator is 0), the domain of f/g is exactly the same as f+ g.
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