Khi α = pi/6 thì biểu thức sin^2 2 α + 4 sin 4 α − 4 sin 2 α . cos 2 α 4 − sin2 2 α − 4 sin 2 α có giá trị bằng.

Chọn C\[\begin{array}{l}\frac{{si{n^2}2\alpha  + 4si{n^4}\alpha  - 4{{\sin }^2}\alpha .{{\cos }^2}\alpha }}{{4 - {{\sin }^2}2\alpha  - 4{{\sin }^2}\alpha }} = \frac{{4si{n^4}\alpha }}{{4(1 - {{\sin }^2}\alpha ) - 4{{\sin }^2}\alpha .{{\cos }^2}\alpha }}\\ = \frac{{si{n^4}\alpha }}{{{{\cos }^2}\alpha (1 - {{\sin }^2}\alpha )}} = \frac{{si{n^4}\alpha }}{{{{\cos }^4}\alpha }} = {\tan ^4}a \Rightarrow {\rm{BT}} = {\tan ^4}\left( {\frac{\pi }{6}} \right) = \frac{1}{9}.\end{array}\]