A Treatise on Plane Trigonometry (Classic Reprint)

Plane Trigonometry. It is hoped that the work will assist in informing and training students of Mathematics who are intending to proceed considerably further in the study of A nalysis, and that, in view of the fulness with which the more elementary parts of the subject have been treated, the book will also be found useful by those whose range of reading is to be more limited. The definitions given inC hapter iii., of the circular functions, were employed by De Morgan in his suggestive work on Double Algebra and Trigonometry, and appear to me to be those from which the fundamental properties of the functions may be most easily deduced in such a way that the proofs may be quite general, in that they apply to angles of all magnitudes. It will be seen that this method of treatment exhibits the formulae for the sine and cosine of the sum of two angles, in the simplest light, merely as the expression of the fact that the projection of the hypothenuse of a right-angled triangle on any straight line in its plane, is equal to the sum of the projections of the sides on the same line. The theorems given inC hapter vii. have usually been deferred until a later stage, but as they are merely algebraical consequences of the addition theorems, there seemed to be no reason why they should be postponed.
(Typographical errors above are due to OCR software and don't occur in the book.)

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