Chapter 8 - Section 8.1 - The Square Root Property and Completing the Square - Exercise Set - Page 593: 23

$(x+1)^2$

Given: $x^2+2x$ We need to add $1^2$ to complete the square. Thus, $x^2+2x+1^2=x^2+2x+1$ Apply sum of polynomial formula. or, $x^2+2(x)(1)+1^2$ Compare it with sum of polynomial formula , we $(a+b)^2=a^2+2ab+b^2$ Hence, we have $(x+1)^2$