RowBox[{Cell[TextData[Theorem: The square root of an integer is either an integer or irrational.]], }]

StyleBox[Cell[Proof:], FontWeight -> Plain, FontSlant -> Italic]

RowBox[{    , StyleBox[RowBox[{n^(1/2), =, , RowBox[{a/b, Cell[], Cell[]}]}], TextAlignment -> Center, FontWeight -> Plain], }]

    n = a^2/b^2

Cell[]

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