  In this paper, we consider the relation between $p > 1$ and critical
dimension of the extremal solution of the semilinear equation
$$\{\begin{array}{lllllll} \beta \Delta^{2}u-\tau \Delta
u=\frac{\lambda}{(1-u)^{p}} & in\ \ B, 0<u\leq 1 & in\ \ B, u=\Delta u=0 & on\
\ \partial B, \end{array} . $$ where $B$ is the unit ball in $R^{n}$,
$\lambda>0$ is a parameter, $\tau>0, \beta>0,p>1$ are fixed constants. By
Hardy-Rellich inequality, we find that when $p$ is large enough, the critical
dimension is 13.}
