有些几何题用代数法求解很方便:将几何量(角、线段等)转化为代数式表示,或用代数式表示其关系,经过整理,得出相应的结论.一、设线段为未知数例1在△ABC中,AB=AC,AD是∠A的平分线.已知△ABC的周长是64,△ABD的周长是56,求AD的长.分析:由题设,应考虑使用“三线合一”的性质.解:由题意画出图1,可设AB=AC=x,BD=CD=y,AD= Some geometrical problems are easy to solve with algebraic methods: transform geometrical quantities (angles, line segments, etc.) into algebraic expressions, or use algebraic expressions to express their relationships, and then get the corresponding conclusions. First, set the line segment to unknown number 1 in △ In ABC, AB=AC, AD is the bisector of ∠A. The circumference of known △ABC is 64, the circumference of △ABD is 56, and the length of AD is determined. Analysis: From the question, you should consider using the “three lines” The nature of “unity”. Solution: Draw the figure 1 from the question, you can set AB = AC = x, BD = CD = y, AD =