... duge dd thi ham sd tren doan [-27t; 27r]. Dd thi ham sd duoc bidu dien tren hinh 5. Hinh 5 11 b) Ta cd X cos— 2 X X cos—, ndu cos— > 0 2 2 X X -cos—, ne'u cos— < 0. ... it360°, it G X = 5° + /:180°, it G Z X = -55° + A:180, it e Z. ôã <ằ 2. BTBS>11-A 17 1 + 2eosx = 0 3 - cosx = 0 d) Ta ed (1 + 2cosx)(3 - cosx) = 0 <» Phuong ... /tl80°, it e » 3x = itl80°, it e Z <=> X = it60°, it G Z. 18 2. BTBS>11-B c) cot 4x -n = ^i^ <» cotj 4x - — J = cot — <:> 4x - — = — + K7t,...