Sea $a \geq b \geq c \geq d>0$ . Demuestra que\n\[\n\frac{b^3}{a} + \frac{c^3}{b} + \frac{d^3}{c} + \frac{a^3}{d} + 3 \left( ab+bc+cd+da \right) \geq \n4 {\left( a^2 + b^2 + c^2 +d^2 \right)}.\n\]