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Published online by Cambridge University Press: 20 January 2009
The theorem in question is that if two of the diagonals of a spherical quadrilateral be quadrantal arcs, the third diagonal is also a quadrantal arc. (Fig. 31.)
Denote the direction cosines of the radius to the point 1 by l1, m1, n1, & c., and l1l2 + m1m2 + n1n2 by 12.