$HPT\Leftrightarrow \left\{\begin{matrix} (x+y)(x^2+y^2-xy)=4(x^2+y^2)(1)\\ (x-y)(x^2+y^2+xy)-6(x-y)(x+y)=0(2) \end{matrix}\right.$
$PT(2)\Leftrightarrow (x-y)[(x+y)(x+y-6)-xy]=0$
$\Leftrightarrow$ $\begin{bmatrix} x=y\\ (x+y)(x+y-6)=xy \end{bmatrix}$