## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$25$, $(x+5)^2$
To complete the square of $x^2 + 10x$, we add $(b/2)^2$. In this case, the "$b$" is the second coefficient, $10$. Thus, we add $(10/2)^2=25$, that is, we have $x^2 + 10x+25$. Then by factorization, we get $$x^2 + 10x+25=(x+5)^2 .$$