What is the difference between the method of finding dihedral angle and plane angle of a straight line? A little dizzy. . .

Line-plane angle: find the projection angle of straight line, diagonal line and diagonal line perpendicular to the plane. Vector method: sin θ = | cos |, where ab is diagonal and n is plane normal vector.

Dihedral angle: find the perpendicular line of the intersection of two planes, and the included angle of the perpendicular line is the plane angle of the dihedral angle. The triple vertical theorem is commonly used. Vector method: cos θ = cos < m, n >, m and n are the normal vectors of two planes. As for acute angle and obtuse angle, see figure.