Osgood, William F. (William Fogg), 1864-1943, Graustein, William Caspar, 1888-1941, joint author · New York, The Macmillan company · leaf 103
bout its axis, and if a source of light be placed at the focus, the reflected rays will all be parallel. This phenomenon is due to the fact that the focal radius FP drawn to any point P of the parabola makes the same angle with the tangent at P as does theline through P parallel to the axis.The… … Fig. 7 MF=xx + But this is precisely the distance of P from D, § 2, andhence, by the definition of the parabola, it is also equal to FP.We have, then, that MF = FP. Consequently, the triangleMFP is isosceles, and y.FMP=y.MPF.But ^.FMP^^SPT, and the proposition is proved. The result can be restated…
Geometry, Analytic
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