Clairaut’s theorem. How to prove that geodesic curvature parallel Radius of the surface of revolution (i. e the value inversed to geodesic curvature) is equal to tangential line segment and meridian closed between the point of tangency and surface axis.enter image description here
I know that I should use the Clairaut's theorem.But I have no idea how to prove it. Can you help me to prove this?