In an isosceles triangle ABC (AB =
BC), the sum of the distances from any point on the extension of
CA to the equal sides is equal to the altitude of the equal
sides.
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The theorem is named for Vincenzo Viviani (16221703), a pupil of Galileo and Torricelli, who is also remembered for a reconstruction of a book on the conic sections of Apollonius and for finding a way of trisecting an angle through the use an equilateral hyperbola.
