Answer
number to be added:$25$
factored form: $(x+5)^2$
Work Step by Step
RECALL:
To complete the square for the expression $x^2+bx$, $(\frac{b}{a})^2$ must be added. The factored form of the result is $(x+\frac{b}{2})^2$.
Thus, in the given expression add $(\frac{10}{2})^2=5^2=25$ to obtain:
$x^2 + 10x + 25$
Writing the square in factored form gives:
$x^2+10x+25 = (x+5)^2$
