Answer
$(-\infty,1]\cup[2,3] $.
Work Step by Step
Step 1. For $(x-1)(x-2)(x-3)\le0$, identify boundary points as $x=1,2,3$.
Step 2. Form intervals $(-\infty,1],[1,2],[2,3],[3,\infty)$.
Step 3. Choose test values for each interval $x=0,1.5,2.5,4$.
Step 4. Test the inequality to get results: $True,\ False,\ True,\ False$
Step 5. We have the solution $(-\infty,1]\cup[2,3] $.
