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立体几何中,有些题目常涉及到线段的中点,同时至少还有一个中点隐含在题设条件中,解题成功与否通常取决于解题者是否找到了这些恰当的中点。因此在解题时,要善于抓住中点,捕捉中点所隐含的解题信息,探寻解题的突破口,从而使问题获解. 例1 如图1,正三棱锥P-ABC各棱长均相等,M为棱AB的中点,则PA与CM所成角的余弦值
In the three-dimensional geometry, some topics often involve the midpoint of the line segment. At the same time, at least one midpoint is implicit in the condition. The success of the problem usually depends on whether the solver finds the appropriate midpoint. Therefore, when solving a problem, we must be good at grasping the midpoint, capture the problem information hidden in the midpoint, and explore the breakthrough point for solving the problem, so as to solve the problem. Example 1 As shown in Figure 1, the length of the regular pyramid P-ABC All are equal, M is the midpoint of the edge AB, then the cosine of the angle formed by PA and CM