Statement I : If $a,b,c$ are in A.P then  $a^3+c^3-8b^3=-6abc$
Statement II : Fifth term of a GP is 2, then the product of its 9 terms is 512 which of the above statements are true 

 $2b=a+c$
 $\Rightarrow 8b^3={(a+c)}^3=a^3+c^3+3ac(a+c)$
Or  $8b^3=a^3+c^3+3ac\left(2b\right)$
Or  $a^3+c^3-8b^3=-6abc$
$\because a_4=2\Rightarrow a{r}^4=2$ 
Now,  $a\times ar\times a{r}^2\times a{r}^3\times a{r}^4\times a{r}^5\times a{r}^6\times a{r}^7\times a{r}^8$
$=a^9{r}^{36}={\left(a{r}^4\right)}^9=2^9=512$