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This derivation . . . 

 uses similar triangles and the Pythagorean Theorem.  While no advanced mathematical concepts are required, the curve provides excellent preparation for studying the calculus of variations.  Let A move along line QA.  Segment OA changes.  The intersection of segment OA and the Circle at B also changes.  Now focus on point P.


Any point  Pon the "witch" is located by constructing lines parallel to the  x and yaxes  through   Band A respectively and determining the point  P of intersection.  The parallel lines are tangent to the circle.
Derivation of the Witch of Agnesi



For further explanation, please see the MAA's College Mathematics Journal.

Gray, S. I. B. and Malakyan, Tagui (September, 1999).  The Witch of Agnesi:  A Lasting Contribution from the First Surviving Mathematical Work Written by a Woman.  A commemorative on the 200th anniversary of her death. College Mathematics Journal, Vol. 30, No.4, 258 - 268.
 

See

 http://www.math.hmc.edu

for a brief review. Search under MAA Journals, Agnesi and 1999.