Cho cos α = 1/4 . Tính cos 2α .18/39Cho \[{\rm{cos}}\alpha \,\,{\rm{ = }}\,\,\frac{1}{4}\]. Tính \[{\rm{cos}}2\alpha \].\[{\rm{cos}}2\alpha = \frac{1}{2}\].\[{\rm{cos}}2\alpha = - \frac{7}{8}\].\[{\rm{cos}}2\alpha = \frac{7}{8}\].\[{\rm{cos}}2\alpha = \frac{9}{8}\].Giải thíchChọn B \[{\rm{cos}}2\alpha = 2{\cos ^2}\alpha - 1 = 2.\frac{1}{{16}} - 1 = - \frac{7}{8}\]