Osgood, William F. (William Fogg), 1864-1943, Graustein, William C. (William Caspar), 1888-1941, joint author · New York : The Macmillan Company · leaf 115
axis, and if a source of light be placed at the focus, the reflected rays will all be parallel. This phenomenon is duetto 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 proof of… … Fig. 7 MF= Xi + m 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 ^ FMP = ^ MPF. But ^F3rP=^SPT, and the proposition is proved. The result can be…
