ĐK sin x cos x # 0, tan x – cot x # 0, cos (2x – π/4) cos (2x + π/4) # 0VT=sin (2x – π/4) sin (2x + π/4) / [cos (2x – π/4) cos (2x + π/4)]<=> [cos (π/2) – cos 4x] / [cos 4x + cos (π/2)]
= -1 (*)
VP= (4cos² 2x) / (sin x / cos x – cos x / sin x)
= (4cos² 2x) sin x cos x / (sin² x – cos² x)
= (4cos² 2x) sin x cos x / (– cos 2x)
= -4 cos 2x sin x cos x
= 2 cos 2x sin 2x
= sin 4x (**)
tu (*)(**) ta co pt da cho tro thanh sin 4x = 1