請問這種題目怎麼解(a-b-c)*(a+b+c)?

(a - b - c) × (a + b + c)
= [a - (b + c)] × [a + (b + c)]
= a² - (b + c)²
= a² - (b² + 2bc + c²)
= a² - b² - c² - 2bc

(a-b-c)(a+b+c)
=[(a-b)-c][(a+b)+c]
=(a-b)(a+b)+c(a-b)-c(a+b)-c^2
=a^2-b^2-c^2

a^2+b^2+c^2+2ab+2ac+2bc