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Conics (2) Everything here follows from the Fundamental Theorem of Projective Geometry. 

Ranges of randomlylocated points in two lines, m and n, may always be projected by points M and N respectively (not in m and n) into two ranges in a third line, i, but, no more than three randomlylocated points in each of the first two lines may be projected into the third line, if the two ranges in the third line, i, are to be the same range. 
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Given our three lines and two Projection Points, and a point P, say, in line m, say, we can always find the corresponding point, P', in line n, and it is of great interest to follow what happens to the line joining these two points as they are moved in their lines. It envelops a conic! In this case, the conic is an ellipse. [Another construction from Nick Thomas' Site] 

The Principle of Duality gives us the Dual Form at once.
