Thank you for your answers! I figured it out these days! The situation of the discussion scope is like this. For example, there are two unknowns in the formula for finding the limit, but one of them is a constant, but the different values of this constant will affect the result of the limit. At this time, it is necessary to discuss the range of this unknown constant. Editor: This article was edited on May 20th, 2009 at 22: 18. ※