72 The base $ABCD$ of the quadrilateral pyramid $P-ABCD$ is a convex quadrilateral, $AC, BD$ intersect at $O$, if the area of $\triangle AOB$ is $36$, the area of $\triangle COD$ is $64$, and the height of the quadrilateral pyramid is $9$, then the minimum volume of such a quadrilateral pyramid is . $\qquad$
$72 \quad 588$.
$$
\begin{aligned}
\frac{1}{2}(O B \times O C+O D \times O A) & \geqslant 2 \sqrt{\frac{O B \times O C \times O D \times O A}{2 \times 2}} \\
& =2 \sqrt{36 \times 64}=2 \times 6 \times 8 \\
& =96 .
\end{aligned}
$$

When $O B=12, O C=8, O D=16, O A=6$, the equality holds, so the minimum base area is
$$
36+64+96=196
$$

The minimum volume is
$$
\frac{1}{3} \times 9 \times 196=588
$$