In a relation (inequality or equation) containing two quantities (a constant and a variable), the range of the constant is required, and the variable and the constant can be separated (that is, the variable and the constant are at one end of the formula), so as to find the range of the constant. This method can be called separation constant method. This method can simplify the answer to the question. For example: Y=(ax+b)/(cx+d), (a≠0, c≠0, d≠0), where a, b, c and d are constants. For example: y=x/(2x+ 1). That is, the molecule does not contain the x term. y = X/(2x+ 1)=[ 1/2 *(2x+ 1)- 1/2]/(2x+ 1)= 1/2。