An elementary treatise on coordinate geometry of three dimensions (1912)
Bell, Robert J. T. (Robert John Tainsh), 1877-1963 · London : Macmillan and co., limited · leaf 41
ut the B.G. B 18 COORDINATE GEOMETRY [CH. II. opposite side, BC, of the triangle ABC, internally. Andtherefore the triangle ABC can always be divided by a linethrough a vertex into two triangles, with a common sideparallel to the given plane ABC, and hence, by (ii),A ABC = COS 9 A ABC. Suppose now… … Fig. 16. Fig. 17. see that if cos 9 is positive, the directions of rotation ABC,ABC have the same sign, and that if cosO is negative,they have opposite signs. That is, the areas have thesame sign if cos 9 is positive, and opposite signs if cos 9is negative. Hence the equation AABC = cosO A ABC…
Geometry, Analytic
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