“算两次”是一种重要的数学方法,又称为富比尼(G.Fubini)原理.它的基本思想是:将同一个量从两个不同角度计算两次,从而建立等量关系.如立体几何中求距离常用的等体积法,就是利用三棱锥可换底的特点,两次计算体积建立等式求高(即距离).又如在解析几何中求某些动点轨迹,常根据动 “Calculate twice ” is an important mathematical method, also known as the G.Fubini principle, whose basic idea is: to calculate the same amount twice from two different angles to establish Volume relations such as the three-dimensional geometry commonly used to find the same volume method, is the use of the characteristics of the triangular pyramid can be replaced by the bottom, calculate the volume twice to find the equation higher (ie, distance) Another example is the analytic geometry to find some of the moving point Track, often based on action