.Đáp ána) Theo định lý Vi-ét, ta có:\(\eqalign{
& \left\{ \matrix{
{x_1} + {x_2} = - 2(\sqrt 3 + 1) \hfill \cr
{x_1}{x_2} = 2\sqrt 3 \,\,\,(\Delta ' > 0) \hfill \cr} \right. \cr
& \Rightarrow x_1^2 + x_2^2 = {({x_1} + {x_2})^2} - 2{x_1}{x_2} \cr&= 4{(\sqrt 3 + 1)^2} - 4\sqrt 3 = 4(4 + \sqrt 3 ) \approx 22,93 \cr} \) b) \(x_1≈ -0, 73;x_2≈ -4, 73\)