1, if the triangle is acute or obtuse, the right triangle 1/2 not necessarily! 2,0 < Sina <1,a/Sina = b/sinb, it doesn't matter! 3, A is an obtuse angle, and from A to BC, at 丄 BC! Then the original formula = at/TB = tagb = b, at/TC = tagc = c, tag (Wu-b-c) =-tag (b+c) = b+c/1+2bc, and the original formula = b+c/1+2bc+. A = 1/2, b = 1, c = 1, Cos Wu/6 = 3/2 = 4 factory 3/8 > COSA, established! 5, a/b obtuse angle! = 1, sina * t < sinb * t * t means a < b * t! T > 1, so wrong, big side to big angle, small side to small angle!