Answer
$\{x|x=2\}$.
Work Step by Step
Step 1. $x^2-4x+4\le0 \longrightarrow (x-2)^2\le0$.
Step 2. As $(x-2)^2\ge0$, the only solution is $(x-2)^2=0$, thus $\{x|x=2\}$.
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