To answer your first question, you can't choose side lengths of 1 or 2 because , so rule 2 isn't satisfied, and , so rule 2 isn't satisfied.
To answer your second question, you need to know about the cosine rule.
You should know that when and therefore . So that means that , since you're subtracting something positive.
However, when , this is a right-angle triangle and an application of Pythagoras' Theorem, and when and therefore . So that means that , since you are subtracting something negative (i.e. adding something).