Let $a$ and $b$ are two numbers.
Then, $A=\frac{a+b}{2} \ldots(i)$
Also, $a, p, q, b$ are in $G P$.
$\frac{p}{a}=\frac{q}{a}=\frac{b}{q}$
$\Rightarrow \frac{p^{2}}{q}=a$ and $\frac{q^{2}}{p}=b$
$\therefore \frac{p^{2}}{q}+\frac{q^{2}}{p}=a+b=2 A$ [Using eq. (i)]