An introduction to projective geometry and its applications; an analytic and synthetic treatment (1905)
Emch, Arnold, b. 1871 · New York, J. Wiley & sons; London, Chapman & Hall, limited · leaf 127
e AFt and AFX include an angle of n. D is therefore the pole of a focal chord A A and, as such, lies in thedirectrix. Hence: The tangents front any point of the directrix of a parabola tothe parabola are perpendicular to each other. It is known from § 33 that the polar involutions around afocus are… … Fig. 46. The portion of a tangent of a conic between its point of tangencyand its intersection with the corresponding directrix appears assubtended by a right angle when seen from the focus. THEORY OF CONICS. 115 Further, from the fact that the polar of a point which is situatedon another polar…
Geometry, Projective · Geometry, Analytic
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