Question

Cho pt \(2017x^2-\left(m-2018\right)x-2019=0\) với m là tham số. Tìm m để pt có hai nghiệm x₁, x₂ thỏa \(\sqrt{x_1^2+2018}-x_1=\sqrt{x_2^2+2018}+x_2\)

Answer 1

Do \(x_1x_2=-\frac{2019}{2017}< 0\Rightarrow\) pt có 2 nghiệm trái dấu.

\(\sqrt{x_1^2+2018}-x_2=\sqrt{x_2^2+2018}+x_1\)

\(\Rightarrow x_1^2+x_2^2+2018-2x_2\sqrt{x^2_1+2018}=x_1^2+x_2^2+2018+2x_1\sqrt{x_2^2+2018}\)

\(\Leftrightarrow-x_2\sqrt{x_1^2+2018}=x_1\sqrt{x_2^2+2018}\)

\(\Rightarrow x_2^2\left(x_1^2+2018\right)=x_1^2\left(x_2^2+2018\right)\)

\(\Rightarrow x_1^2=x_2^2\Rightarrow x_1=-x_2\) (do \(x_1;x_2\) trái dấu)

\(\Rightarrow x_1+x_2=0\Rightarrow\frac{m-2018}{2017}=0\Rightarrow m=2018\)