sin3x−cos3x=sinx+cosx
$\Leftrightarrow \sin x (1 - \sin^2 x) + \cos x(1+\cos^2 x) = 0$
$\Leftrightarrow \sin x \cos^2 x + \cos x (1+\cos^2 x) = 0$
$\Leftrightarrow \cos x(\sin x \cos x +1 + \cos^2 x ) = 0$
$\Leftrightarrow \left [ \begin{matrix} \cos x = 0 \\ \cos^2 x + \sin x \cos x + 1 = 0\ \ (1) \end{matrix} \right.$
$(1) \Leftrightarrow 1+\tan x + \dfrac{1}{\cos^2 x} = 0$
$\Leftrightarrow 1 + \tan x + \tan^2 x + 1 = 0$ dễ rồi