Question 271677
Find all numbers for which the rational expression is undefined:
s^3-9s/s^2-4 
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If you divide anything by a zero, it is considered "undefined".  That is, when the denominator equals zero your rational expression will be undefined.
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So, if we set the denominator (s^2-4) to zero and solve for s we'll find your answer:
s^2-4 = 0
s^2 = 4
s = sqrt(4)
s = {-2,2}  (this is what they are looking for)