Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-oxford/2015/prelims/mathematics-iii/4/a/i/solution

The events form a partition of a sample space when they are pairwise disjoint sets and cover the whole sample space:
Thus exactly one of these events occurs on every outcome. Under the usual set partition convention the parts are nonempty; empty parts can simply be discarded.

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