= a² - b² - c² - 2bc - 2a² - 2ab – 2ac = - a² - b² - c² - 2bc - 2ab – 2ac
= -(a² + b² + c² + 2ab + 2ac + 2bc) = -(a+b+c)² = -10²= -100
= -(a² + b² + c² + 2ab + 2ac + 2bc)² = -(a+b+c)² = -10²= -100