Wentworth, G. A. (George Albert), 1835-1906 · Boston, Ginn & company · leaf 114
(Fig. 36) be such a right triangle, and let A, B, Cdenote the angles of the tri-angle ; a, b, c, respectively, theopposite sides. Let 0 be the right angle, andfor the present suppose thateach of the other parts is lessthan 90°. Let planes be passed throughthe sides, intersecting in theradii OA, OB,… … 106 TRIGONOMETRY. B BE and DE are each ±OA (Geom. § 454); therefore ABED = A. The plane BDE is J_ theplane^O(7(Geom.§472); henceBD, which is the intersectionof the planes BDE and BOC, is± the plane ^OC(Geom. § 475),therefore JL 00 and DE. Nowcose = OE= OD X cos&,C and OD = cos a. .. cose = cos…
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