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Theorem/proof box template

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<strong>THEOREM/LEMMA/PROPOSITION:</strong> STATEMENT OF THEOREM
$$\mathscr{L}\{\alpha f + \beta g\} = \alpha \mathscr{L}\{f\} + \beta \mathscr{L}\{g\}.$$
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<strong>Proof: proof goes here</strong> █ 
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creates

THEOREM/LEMMA/PROPOSITION: STATEMENT OF THEOREM $$\mathscr{L}\{\alpha f + \beta g\} = \alpha \mathscr{L}\{f\} + \beta \mathscr{L}\{g\}.$$

Proof: proof goes here █