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

$$x^2+2x+1=(x+1)^2 .$$
Since the polynomial $x^2+2x+1$ is of the form $a^2+2ab+b^2=(a+b)^2$, by putting $a=x$, $b=1$, we have $$x^2+2x+1=(x+1)^2 .$$