X approaching 0 means that X approaches 0 from a place greater than 0, so if calculated by the expression of X greater than 0, X approaching 0 means that X approaches 0 from a place less than 0. When the two are not equal, the limit that X approaches 0 does not exist. When the two are equal, the limit at which x approaches 0 is that number. In fact, it is good to be familiar with the definitions of function continuity and discontinuity.