Bài 1.
(x - y)² - (x+y)²
= [(x - y) - (x + y)] [(x - y) + (x + y)]
Bài 2.
a) (x + y)² - y² = x (x + 2y)
⇔ (x + y - y) (x + y + y) = x (x + 2y)
⇔ x (x + 2y) = x (x +2y) (ĐPCM)
b) (x² + y²)² - (2xy)² = (x + y)² × (x - y)²
⇔ (x² + y² - 2xy) (x² + y² + 2xy) = (x + y)² × (x - y)²
⇔ (x - y)² × (x + y)² = (x + y)² × (x - y)² (ĐPCM)