\[\boxed{\mathbf{9}.}\]
\[1)\ x^{8} - 4 = (x^{4} - 2)(x^{4} + 2)\]
\[2)\ 25n^{2} - 49p^{4} =\]
\[= \left( 5n - 7p^{2} \right)\left( 5n + 7p^{2} \right)\]
\[3)\ 1\frac{9}{16}a^{2} - 0,09b^{2} = \frac{25}{16}a^{2} -\]
\[- 0,09b^{2} =\]
\[= \left( \frac{5}{4}a - 0,3b \right)\left( \frac{5}{4}a + 0,3b \right)\]
\[4)\ 0,0081x^{6} - 1\frac{7}{9}y^{10} =\]
\[= 0,0081x^{6} - \frac{16}{9}y^{10} =\]
\[= \left( 0,09x^{3} - \frac{4}{3}y^{5} \right)\left( 0,09x^{3} + \frac{4}{3}y^{5} \right)\]