Solve the quadratic equation by completing the square:

$x^2-4ax+4a^2-b^2=0$

Find the roots of the quadratic equation: ${a^{2}x}^2-3abx+{2b}^2=0$ (if they exist) by the method of completing the square.

Solve, by completing the square:

$x^2-\left(\sqrt{2}+1\right)x+\sqrt{2}=0$

Find the discriminant of the quadratic equation: $x^2-2x+k=0$