a: Sai
c: \(2x^2-x-5=0\)
=>\(x^2-\frac12x-\frac52=0\)
=>\(x^2-\frac12x+\frac{1}{16}-\frac{41}{16}=0\)
=>\(\left(x-\frac14\right)^2=\frac{41}{16}\)
=>\(\left[\begin{array}{l}x-\frac14=\frac{\sqrt{41}}{4}\\ x-\frac14=-\frac{\sqrt{41}}{4}\end{array}\right.\Rightarrow\left[\begin{array}{l}x=\frac{\sqrt{41}+1}{4}\\ x=\frac{-\sqrt{41}+1}{4}\end{array}\right.\)
\(x_1
=>\(x_1=\frac{-\sqrt{41}+1}{4};x_2=\frac{\sqrt{41}+1}{4}\)
\(x_1^2+x_2^2=\left(\frac{-\sqrt{41}+1}{4}\right)^2+\left(\frac{\sqrt{41}+1}{4}\right)^2=\frac{42-2\sqrt{41}}{16}+\frac{42+2\sqrt{41}}{16}=\frac{84}{16}=\frac{21}{4}=\left(\frac{\sqrt{21}}{2}\right)^2\)
=>Đúng
d: Đúng
