Although trigonometry is a branch of mathematics which, at first sight, appears to be exclusively concerned with the study of the sides and angles of triangles, it is, in fact, much more fundamental than that, and a knowledge of it is quite indispensable in many branches of economics and mathematics which few would consider to deal simply with triangles. In particular, it is important in studying cyclical fluctuations. In this chapter we shall introduce the simpler ideas, and then list some of the more important results, without bothering to encumber the reader with proofs.
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