Groups We have a group of 10 students that we want to partition into two sub-groups. None of the two subgroups can be empty. First, explain why the number of ways we can do this is 9 r%3D1 Explain all...

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Extracted text: Groups We have a group of 10 students that we want to partition into two sub-groups. None of the two subgroups can be empty. First, explain why the number of ways we can do this is 9 r%3D1 Explain all parts of the expression. Second, give a closed form for the expression.