pe petrovalv a² : (a - b)² - (a + b) : 2(a - b) = = 2a² : 2(a - b)² - (a + b)(a - b) : 2(a - b)² = = 2a² : 2(a - b)² - (a² - b²) : 2(a - b)² = = (2a² - a² + b²) : 2(a - b)² = = (a² + b²) : 2(a - b)²