有些数学题,初看起来似乎图形不定、变量众多,让人感觉难以求解,但我们可以用“以不变应万变”来应对,发掘问题中某些量的不变性,往往可寻找出确定的数量关系或位置关系,从而找到解题思路.现举例如下:一、形状变化,角度不变例1如图1,边长为2的正△ABC中,D是BC边上的任一点,连结AD,以AD为直径作⊙O,交AB于点E,交AC于点F,连结EF,求EF的最小值. Some mathematics questions may seem at first glance with uncertain figures and many variables, which make people feel difficult to solve. However, we can deal with “invariable changes” to explore the invariability of certain quantities in the problems and we can often look for them. Determine the quantitative relationship or positional relationship to find a solution to the problem. Here are some examples: 1. Change in shape, angle unchanged Example 1 In Figure 1, a positive △ ABC with a side length of 2, D is the one on the BC side. One point, link AD, use AD as diameter for ⊙O, cross AB at point E, AC at point F, link EF, find the minimum value of EF.