Answer
To complete the square for the expression $x^{2}+bx$, add $(\displaystyle \frac{b}{2})^{2}$ so that the expression becomes a perfect square trinomial.
Work Step by Step
$x^{2}+bx+(\displaystyle \frac{b}{2})^{2}=(x+\frac{b}{2})(x+\frac{b}{2})=(x+\frac{b}{2})^{2}$
