Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2020/ii/paper-2/18g/c/i/solution

A group is solvable when its derived series
eventually reaches the trivial group; equivalently, it has a subnormal series with abelian quotients.
The symmetric group is solvable because
has abelian quotients , , and , where is the Klein four-group. Thus

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