Wentworth, G. A. (George Albert), 1835-1906 · Boston, Ginn & company · leaf 43
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] … In this proof x and y, and also the sum x + y, are assumedto be acute angles. If the sumx + y of the acute angles # andy is obtuse, as in Fig. 51, theproof remains, word for word,the same as above, the only dif-ference being that the sign ofOF will be negative, as DG isnow greater than OE. The…
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