[tex] \it P\in a\ \ \ \ \ \ (1)
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P\in b\ \ \ \ \ \ (2)
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(1), (2) \Rightarrow a\cap b =\{P\} \ \ \ \ \ (3)
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Dar, problema\ afirm\breve{a}\ c\breve{a}:\ \ a\cap b =\{Q\} \ \ \ \ \ (4)
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(3),\ (4) \Rightarrow P = Q (cele\ dou\breve{a} \ puncte\ coincid) [/tex]