(x² + 1)² - 4x² = (x² + 1)² - (2x)² = (x² + 1 - 2x)(x² + 1 + 2x) =
= (x² - 2x + 1)(x² + 2x + 1) = (x - 1)²(x + 1)² = (x - 1)(x - 1)(x + 1)(x + 1)
(x² + 1)² - 4x² = (x² + 1)² - (2x)² = (x² + 1 - 2x)(x² + 1 + 2x) =
= (x² - 2x + 1)(x² + 2x + 1) = (x - 1)²(x + 1)² = (x - 1)(x - 1)(x + 1)(x + 1)