Sum of 2 Acute Angles Small 320×139 Medium 640×279 Large 1024×446 Download Tiff Original 2400×1045 | (178.4 KB) Description Illustration of two triangles, showing the sine of the sum of two acute angles expressed in terms of the sines and cosines of the angles. Keywords angle, cos, cosine, sin, Sine, triangle, triangles, trig functions, trigonometric, trigonometry Galleries Trigonometry: Angles Source: George N. Bauer, PH.D. and W.E. Brooke, C.E., M.A. Plane And Spherical Trigonometry (Boston: D.C. Heath & Co., 1917) 61 Courtesy the private collection of Roy Winkelman