I agree with undefined. If you wanted a clever proof (ie, one that doesn't use the fact that the asquare root of 4 is known to be 2), you need to say so.
@Mukilab: Your most recent post highlights the difference between "X is cheating" and "X feels like cheating." Maybe what I should have said is that my proof shouldn't feel like cheating. When there is a simple proof available, why use a more complex one?
undefined @ 13: Well, some people might have a problem with , if they're using equality as being the same thing as identity. 5 is not identically equal to plus or minus 5. In effect, what I'm really doing here is a short-hand notation for , where this equality does mean identity.
I think universally stands for . Reference (Wikipedia - Plus-minus sign - Subheading: Shorthand).
As long as you present a valid proof, you should get full marks. I think it would be unreasonable for a teacher/professor/grader to grade otherwise. The type of proof should only matter if the problem statement specifies a particular type of proof.