Question

What number would you add to complete the square for the equation below?x2 + 10x = 0?

Answer 1

For a quadratic of the form $x^2+bx=-c$, we complete the square by first adding 
$(\dfrac{b}{2})^2$
to each side.

Here, the coefficient b is equal to 10. We take half of this—5—and square it to get 25. We add this to both sides of the equation.

FYI, this equation can easily be factored, so there's no need to complete the square.