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Determine when an expression is undefined

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Determine when the following expression is undefined: 2x+9 / x^2-5x-24
POSTED BY: Kojack Davis
Answer
7 months ago
One way of finding out the largest possible domain of that function, i.e. where the expression is defined is to use Wolfram Alpha within Mathematica:
= domain of (2x+9)/(x^2-5x-242x+9)
I added parenthesis to that expression, because I suspect that is what is meant.  
If you click on the little plus symbol next to the solution, that comes up, you will get further information which might be useful.
I do believe that there is no function that does this directly in Mathematica 9, but I might be mistaken. Also, from hearsay I gather that there will be such a function in Mathematica 10. 
POSTED BY: Marco Thiel
Answer
7 months ago
Union[ x /. Solve[Denominator[Together[2 x + 9/x^2 - 5 x - 24]] == 0, x]]

(* Out[336]= {0} *)
POSTED BY: Daniel Lichtblau
Answer
7 months ago