Q15 (a) (iii)

$a>b>c>0$
$ab>b^2>bc>0$
$ab>bc \qquad \text{I}$

$a>b>c>0$
$ac>bc>c^2>0$
$ac>bc \qquad \text{II}$

$a^2+5b^2+9c^2\ge 3(ab+bc+ac) \qquad \text{From Part ii)}$
$\ge 3(bc+bc+ac) \qquad \text{From I}$
$\ge 3(bc+bc+bc) \qquad \text{From II}$
$=9bc$

$\therefore\ a^2+5b^2+9c^2\geq 9bc$

1 – using given condition to setup the 3 inequalities or equivalent merit
1 – correct setting and result