三垂线定理及其逆定理是立体几何中的两个重要定理,在证明线线垂直,用向量来解决立体几何问题时,相对简洁.题如果一个角所在平面外一点到角的两边的距离相等,那么这点在平面内的射影在这个角的平分线上. The three vertical line theorem and its inverse theorem are two important theorems in three-dimensional geometry. When it is proved that the line is vertical and the vector is used to solve the three-dimensional geometric problem, it is relatively concise. If a corner is outside the plane, it is a distance from two sides of the corner. Equal, then the projection in the plane is on the bisector of this angle.