Derive quadratic formula I cannot understand how quadratic formula to solve for $x$ was derived.
On this website, it explains the steps
Following I understand

but I cannot understand how they got 
$b/2a$
and why they are squaring it
$b^2/4a^2$
Really, I am baffled!

$x^2+\frac{b}{a}x+K^2=(x+K)^2$
$x^2+\frac{b}{a}x+K^2=x^2+2xK+K^2$
$\frac{b}{a}x=2xK$
$k=\frac{b}{2a}$
Then we know that
$x^2+\frac{b}{a}x+(\frac{b}{2a})^2=(x+\frac{b}{2a})^2$
$x^2+\frac{b}{a}x+\frac{b^2}{4a^2}=(x+\frac{b}{2a})^2$
This process is called completing the square.
Now we can factor a perfect square. Is this answer sufficient?
