--------- ---------

		The tangent at O is perpendicular to the plane

		>--->--->--->--->   <<---<<---<<---<<
				 \ /
			         / \
		<---<---<---<---<   >>--->>--->>--->>

		Let A and B the two points projected at M. One
		point belongs to the segment on the left, and the
		other to the segment on the right. Both AB and
		f'(B)-f'(A) are perpendicular to the plane

				\  |
				 \ |
				   | \
				   |  \

		Three points projected at the same point of the plane

Charilaos Aneziris, aneziris@hades.ifh.de COPYRIGHT STATEMENT Educational institutions are encouraged to reproduce and distribute these materials for educational use free of charge as long as credit and notification are provided. For any other purpose except educational, such as commercial etc, use of these materials is prohibited without prior written permission.