4ab - a² - b²
= - (a² - 4ab + b²)
= - (a² - 2ab + b²) + 2ab
= - (a - b)² + [√(2ab)]²
= [√(2ab) + (a - b)][√(2ab) - (a - b)]
= [√(2ab) + a - b][√(2ab) - a + b]