# How do you find the missing value to complete the square x^2+2x?

${\left(x + a\right)}^{2} = {x}^{2} + 2 a x + {a}^{2}$
We want $2 a x$ to match $2 x$, so $a = 1$ and the missing value is ${a}^{2} = {1}^{2} = 1$
${x}^{2} + 2 x + 1 = {\left(x + 1\right)}^{2}$