tồn tại x1 ; x2=> k thuôc (-vc;-2]U[2;vc)
tồn tại x1,2<>0 ; f(0)<>0<=> luôn đúng => k thuôc (-vc;-2]U[2;vc)
\(A=\left(\dfrac{x_1}{x_2}\right)^2+\left(\dfrac{x_2}{x_1}\right)^2=\left(\dfrac{x_1}{x_2}+\dfrac{x_2}{x_1}\right)^2-2\)
\(A=\left(\dfrac{x^2_1+x^2_2}{x_1.x_2}\right)^2-2=\left(\dfrac{\left(x_1+x_2\right)^2-2x_1.x_2}{x_1.x_2}\right)^2-2\)
\(A\ge3\Leftrightarrow\left(\dfrac{\left(x_1+x_2\right)^2}{x_1.x_2}-2\right)^2\ge5\)\(\Leftrightarrow\left[{}\begin{matrix}\dfrac{\left(x_1+x_2\right)^2}{x_1.x_2}-2\ge\sqrt{5}\left(1\right)\\\dfrac{\left(x_1+x_2\right)^2}{x_1.x_2}-2\le-\sqrt{5}\left(2\right)\end{matrix}\right.\)
(1) \(\dfrac{\left(2k\right)^2}{4}\ge2+\sqrt{5}\Leftrightarrow k^2\ge2+\sqrt{5}\Rightarrow k\in(-\infty;-\sqrt{2+\sqrt{5}}]U[\sqrt{2+\sqrt{5}};+\infty)\)
(2)<=> \(\dfrac{\left(2k\right)^2}{4}\le2-\sqrt{5}\Leftrightarrow k^2\le2-\sqrt{5}\left(l\right)\)
kết hợp nghiệm \(k\in(-\infty;-\sqrt{2+\sqrt{5}}]U[\sqrt{2+\sqrt{5}};+\infty)\)
