 




O
The tangent at O is perpendicular to the plane
>>>>> <<<<<<<<
\ /
M
/ \
<<<<< >>>>>>>>
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
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Three points projected at the same point of the plane
Charilaos Aneziris, aneziris@hades.ifh.de
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