Show that if a,b and c d are positive ( and c and d are rational), then $\displaystyle (a^c-b^c)(a^d-b^d) \geq 0$ Can we proceed by multiplying the factors like :

$\displaystyle a^{c+d}-b^ca^d-a^cb^d+b^{c+d} \geq 0$ Can somebody brief me the concept behind this..... I will be greatful....