If we think of a cone to be made up of triangles and since a cone has a single point on one of the tappered side,we do not usually define a normal there ,but how can we make opengl to render the triangles which include the vertex ,so that its shading looks fine
Generally, you wouldn’t share the normal for the top of the cone because it’d look wrong no matter how you oriented it; instead, you’d have a vertex for every triangle that reaches the top so the normal of the point is the same as the rest of the side.
thank 
that was helpful