“斜线和平面所成角为斜线和斜线在平面上的射影所成的角,且为斜线和平面内过斜足的所有直线所成角中的最小角”.于是,求解线面角关键是找或做斜线在面上的射影,如何寻求斜线在面上的射影? “The angle formed by the slash and the plane is the angle formed by the projection of the slash and slash on the plane, and is the smallest of the angles formed by the slash and all the lines of the inclined foot in the plane”. The key to solving the line face angle is to find or do a slash on the surface of the project, how to seek the slash on the surface of the project?