\[a^{2} + 8ab + 16b^{2} - 1 =\]
\[= \left( a^{2} + 8ab + 16b^{2} \right) - 1 = (a + 4b)^{2} - 1^{2} =\]
\[= (a + 4b - 1)(a + 4b + 1)\]