It is an obtained solution to an equation which makes the original equation undefined.
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When you say "which," it sounds like it should be multiple choice. Anyways, here's the simplified form of 
(When simplifying fractions, you should <em>never</em> have a square root on the bottom. Multiply by the square root to cancel it out)
(When simplifying fractions, you should <em>never</em> have a square root on the bottom. Multiply by the square root to cancel it out)