$\displaystyle 4x^2-4\sqrt{5x}+5$
I assume this is actually
$\displaystyle 4x^2 - 4\sqrt{5}x + 5$.
Remember that $\displaystyle (a + b)^2 = a^2 + 2ab + b^2$.
What you have posted is a perfect square, because
$\displaystyle 4x^2 - 4\sqrt{5}x + 5 = (2x)^2 + 2(2x)(-\sqrt{5}) + (-\sqrt{5})^2$
$\displaystyle = (2x - \sqrt{5})^2$