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Math Help - Domain and function True/False ?

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    Domain and function True/False ?

    True or false,
    The domain of the function f(x) = x^2-9/x is {X|X /= +/-3}. I don't understand this question. Also, what does X|X mean? Thanks.
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    Re: Domain and function True/False ?

    Quote Originally Posted by kmerr98277 View Post
    True or false,
    The domain of the function f(x) = x^2-9/x is {X|X /= +/-3}. I don't understand this question. Also, what does X|X mean? Thanks.
    \{X|~X\ne\pm 3\} is read as The set of all X such that X is not equal to plus or minus three.
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  3. #3
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    Re: Domain and function True/False ?

    It is, however, NOT true that the domain of (x^2- 9)/x is the "set of all x such that x is not equal to plus or minus 3". The (natural) domain of a function is the set of all values of x for which the formula can be calculated. There is no problem with x= 3 or -3: f(3)= 0/3= 0 and f(-3)= 0/(-3)= 0. There is a problem with x= 0 because then the denominator is 0 and we cannot divide by 0. The domain of (x^2- 9)/x is \{x | x\ne 0\}.

    If the the problem were were with f the reciprocal, f(x)= \frac{x}{x^2- 9}, then, because x^2- 9= (x- 3)(x+ 3), the denominator would be 0 at x= 3 or x= -3 and the domain would be \{x| x\ne \pm 3\}.
    Last edited by HallsofIvy; July 10th 2012 at 03:02 PM.
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