Let A be a point on the surface, the projection of a straight line AO on the surface through A is AB, and AC is a straight line on the surface, then the cosine relation of triangle ∠ OAC, ∠ BAC and ∠ OAB is:

Cos ∠ OAC = cos ∠ BAC× cos∠OAB (cos ∠ BAC and cos ∠ OAB can only be acute angles).

Generally speaking, the angle between the diagonal of cos (OAC) and the plane line = the angle between the diagonal projection of COS and the plane line (BAC) and the diagonal projection of XCOS (OAB). Also known as the minimum angle theorem or claw theorem, it can be used to find the minimum included angle between diagonal and plane straight line.