Đáp án:

 ⇒ $D = 12a^2 - 8ac - 2bc + b^2 + c^2$

-Giải thích:

$D = (3a - b - c)^2 + (2a + b - c)^2 - (a - b - c)^2$

$D = 9a^2 - 6ab - 6ac + b^2 + 2bc + c^2$

=$+ 4a^2 + 4ab - 4ac + b^2 - 2bc + c^2$

=$- a^2 + 2ab + 2ac - b^2 - 2bc - c^2$

$D = (9a^2 + 4a^2 - a^2)$

=$+ (-6ab + 4ab + 2ab)$

=$+ (-6ac - 4ac + 2ac)$

=$+ (2bc - 2bc - 2bc)$

=$+ (b^2 + b^2 - b^2) + (c^2 + c^2 - c^2)$

⇒ $D = 12a^2 - 8ac - 2bc + b^2 + c^2$

#muzzas#