$$\displaystyle \begin{aligned} pt \Leftrightarrow & sin^3x + cos^3x = 2sinx(cosx-sinx)(cosx+sinx) \\ \Leftrightarrow & (sinx+cosx)(1-sinx.cox) = 2sinx.(cosx-sinx)(cosx+sinx) \\ \Leftrightarrow & \left[ \begin{array}{} sinx + cosx = 0 \\ 3tan^2x - tanx + 1 = 0 \ \ (VN) \end{array} \right. \\ \Leftrightarrow & sinx + cosx = 0 \\ \Leftrightarrow & tanx = - 1 \\ \Leftrightarrow & x = -\frac{\pi}4 + k\pi, \ \ \ (k\in \mathbb{Z}) \end{aligned}$$