Question

Factorise the following by the difference of two squares:
x² + y² - z² - 2xy

Answer 1

x² + y² - z² - 2xy
= x² + y² - 2xy - z²
= (x² + y² - 2xy) - z²
= (x - y)² - (z)²
= (x - y - z)(x - y + z).