... x=arctan(-2) + kπ e) cos2x + 5cosx = 2cos2x -1 + 5cosx - = (0,25đ) 2cos2x + 5cosx – = (0,25đ) cosx=1 ⇔ (0, 5ñ ) ⇔x=k 2π( k ∈¢ )(0, 5ñ ) cosx=- π π π 3cosx= ⇔ sinxcosx= (0, 25 ñ ) ⇔cos ... (sai) cosx =0 nghiệm.(0,25đ) cosx ()4tan – 5tanx + = 0(0,5đ) π t anx=1 x= +kπ (0, 25ñ ) ⇔ (k ∈ )(0, 25ñ ) ¢ anx= t x=arctan +kπ e) cos2x + 3cosx = 2cos2x -1 + 3cosx ... (0,5đ) cosx=1 2π 2π ⇔ (0,25ñ ) ⇔ x = ± + k 2π( k ∈¢ )(0, 25ñ ) (0,5ñ ) ⇔cos x=- ⇔cos x = cos 3 cosx=3 d) 2sin2x 5sinx.cosx cos2x = 2 4sin2x – 5sinx.cosx+cos 2x = (0,25đ) () cosx =...