人教版高中课程标准实验教科书·数学选修2-1第106页例2:如图1,甲站在水库底面上的点A处,乙站在水坝斜面上的点B处,从A、B到直线l(库底与水坝的交线)的距离AC和BD分别为a和b,CD的长为c,AB的长为d.求库底与水坝所成二面角的余弦值.此例是一个实际应用问题,难度较低,一般不易引起人们的注意,但笔者认为这样的一个空间图形会衍生出一系列高考命题,颇有回归课本探源,提高数 Example 2: As shown in Figure 1, station A is at point A on the bottom of the reservoir, point B is at point B on the slope of the dam, and from point A to B The distance AC and BD to a straight line l (the intersection of the reservoir bottom and the dam) are a and b respectively, the length of the CD is c, and the length of AB is d. Find the cosine of the dihedral angle between the reservoir bottom and the dam. Case is a practical application, the difficulty is low, the general is not easy to arouse people’s attention, but I believe that such a space will be derived from a series of graphic graphics college entrance examination, rather return to the source of textbooks to improve the number