Wentworth, G. A. (George Albert), 1835-1906 · Boston, Ginn & company · leaf 43
54 PLANE TRIGONOMETRY 7) 77 Now = sin x ; hence, DE = sin x X OD = sin x cos y. And —— = cos x ; hence, CG = cos cc X CD = cos # sin y.Therefore,sin(x + y) = sinxcosy + cosxsiny. [4] Again, cos (x + 3/) = OF = OE — DG.OEODOE = cos x X OD = cos x cos y. … cos x; hence, DG . , . . ——■ = sin a;; hence, DG = sin as X CD = sin z sin y. CD ^ Therefore, cos (x + y) = cos x cos y — sin x sin y. [5]
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