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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
10 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
10 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
10 months ago