(1)^2=> a^2+b^2+c^2+2ab+2bc+2ac=0
=> ab+bc+ac=-2
(...)^2=4
(ab)^2+(bc)^2+(ac)^2=4
(2)^2=>A+2(ab)^2+2(bc)^2+2(ac)^2=16
A=16-4=12
nhầm giờ mới có máy tính
\(\left(a^2+b^2+c^2\right)^2=A+2\left(\left(ab\right)^2+\left(bc\right)^2+\left(ac\right)^2\right)=16\)
\(A=16-2.4=8\)