my book gave me this without any further explanation. how do we get 'when '??
could be something simple that i overlooked >.< please help
The short answer is that the square root term approaches infinity and since a <= 0 we have a situation where - (-x) = +x which means -ax > 0 which leaves infinity + infinity - constant = infinity.
What you may want to do however is use L'hopitals rule: have you used this before?