Napier's rules

Napier's rules

[′nā·pē·ərz ‚rülz]
(mathematics)
Two rules which give the formulas necessary in the solution of right spherical triangles.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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Right spherical triangles allow the use of a set of simplified trigonometric equations generated by Napier's rules. Note that the five-segmented disk in Figure 4 includes all of the parts of the right spherical triangle, except the 90[degree] angle.
A few example formulas generated by Napier's rules follow.