$a,\,b,\,c,\,d> 0,\,a^{\,3}+ b^{\,3}+ 3\,ab= c+ d= 1$$.$ Chứng minh rằng$:$
$$\left ( a+ \frac{1}{a} \right )^{\,3}+ \left ( b+ \frac{1}{b} \right )^{\,3}+ \left ( c+ \frac{1}{c} \right )^{\,3}+ \left ( d+ \frac{1}{d} \right )^{\,3}\geqq \frac{125}{2}$$
M O
$\it{2003}$