在给定的条件下求分式的值,很少是直接代入求值,需要根据题目的特点,将已知条件或所求分式适当变形,再求解.常用的变形方法大致有以下几种: Under the given conditions, the value of fractional expression is seldom directly substituted into the value. According to the characteristics of the subject, the known condition or the required fractional expression should be properly deformed and then solved. The commonly used deformation methods are as follows :