a^4 - 12a^2 + 16 = a^4 - 12a^2 + 36 - 20 = (a^2 - 6)^2 - 20 = (a^2 - 6 - 2√5)(a^2 - 6 + 2√5) = (a - √(6 - 2√5))(a + √(6 - 2√5))(a - √(6 + 2√5))(a + √(6+2√5))
