We have
(a + b)² = a² + 2ab + b²
so that with a = x² and b = 5, we have
x⁴ + 10x² + 25 = (x² + 5)²
Next, we have
a² - b² = (a - b) (a + b)
so that with a = x and b = √5 i,
x² + 5 = x² - (-5) = (x - √5 i) (x + √5 i)
So, the complete factorization over the complexes is
(x - √5 i)² (x + √5 i)²
