Understand that simply adding |A| + |B| double-counts the overlap, so the correct union size subtracts the intersection exactly once: |A ∪ B| = |A| + |B| − |A ∩ B|.
Inclusion-exclusion: |A ∪ B| = |A| + |B| − |A ∩ B|Constraint: |A ∩ B| ≤ min(|A|, |B|)Set |A| = 10, |B| = 10, and |A ∩ B| = 0 (no overlap). Confirm the union size is exactly 20.
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