大家知道:以二面角的棱上任意一点为端点,在两个面内分别作垂直于棱的两条射线,这两条射线所成的角就叫做二面角的平面角.而二面角的大小是用它的平面角来度量的.可见构造二面角的平面角是计算二面角的大小的关键.进一步透视概念,我们又感到:选择端点构造可解三角形,便是解题的突破口. Everyone knows that: With any point on the edge of a dihedral angle as an endpoint, two rays perpendicular to the edge are made in two planes respectively. The angle formed by these two rays is called the planar angle of the dihedral angle. The size of the corner is measured by its plane angle. The plane angle of the visible dihedral angle is the key to calculating the size of the dihedral. With further perspective concepts, we feel that choosing the endpoint structure to solve the triangle is the solution to the problem. Breakthrough.