Answer
$x^2-2x+1
\Rightarrow
(x-1)^2
$
Work Step by Step
The third term of a perfect square trinomial is equal to the square of half the coefficient of the middle term. Hence, to complete the square of the given expression $
x^2-2x
,$ the third term must be
\begin{array}{l}\require{cancel}\left( \dfrac{-2}{2}
\right)^2\\\\=\left(
-1
\right)^2\\\\=
1
.\end{array}
Using $a^2\pm2ab+b^2=(a\pm b)^2$, then
\begin{array}{l}\require{cancel}
x^2-2x+1
\Rightarrow
(x-1)^2
.\end{array}