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voved vo teorijata na mno@estvata i matemati^kata logika

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Koristej}i komplement <strong>na</strong> mno`estva, simetri~<strong>na</strong>ta razlika <strong>na</strong><br />

mno`estva mo`e da se definira i <strong>na</strong> poi<strong>na</strong>kov <strong>na</strong>~in, imeno so<br />

M+N = (M'∩N) ∪ (M∩N'). Za da poka`eme deka dvete definicii opredeluvaat<br />

isto mno`est<strong>vo</strong>, }e go iskoristime sledno<strong>vo</strong> s<strong>vo</strong>jst<strong>vo</strong>:<br />

6.22 o (i) A \ B=A ∩ B';<br />

(ii) A+B = (A \ B) ∪ (B \ A).<br />

Dokaz: (i) x∈A \ B ⇔x∈A ∧ x∉B ⇔ x∈A ∧ x∈B' ⇔ x∈A ∩ B'.<br />

(ii) A+B = (A∪B) \ (A∩B) = (A∪B) ∩ (A∩B)' = (A∪B) ∩ (A'∪B') =<br />

=((A ∪ B) ∩ A') ∩ ((A ∪ B) ∩ B') =<br />

= ((A ∩ A') ∪ (B ∩ A')) ∪ ((A ∩ B') ∪ (B ∩ B')) =<br />

=(A' ∩ B) ∪ (A ∩ B') = (A \ B) ∪ (B \ A). ■<br />

Od <strong>na</strong>rednite tvrdewa sleduva deka operacijata + e asocijativ<strong>na</strong> i<br />

deka va`i distributivniot zakon <strong>na</strong> ∩ <strong>vo</strong> odnos <strong>na</strong> +.<br />

6.23 o X+(Y+Z) = (X+Y)+Z. ■<br />

6.24 o X ∩ (Y+Z) = (X ∩ Y)+(X ∩ Z). ■<br />

1.6.1. Ve`bi:<br />

1. Da se doka`e:<br />

(a) A ∩ (B ∪ C) ⊆ (A ∩ B) ∪ C;<br />

(b) A ∪ (B ∩ C) = (A ∪ B) ∩ (A ∪ C);<br />

(v) A ∩ (B ∪ C) = (A ∩ B) ∪ (A ∩ C).<br />

2. Da se doka`e:<br />

(a) A \ (B ∩ C) = (A \ B) ∪ (A \ C);<br />

(b) A \ (B ∪ C) = (A \ B) ∩ (A \ C);<br />

(v) B(A ∩ B)=B(A) ∩ B(B).<br />

3. X ∩ Y=X ∩ Z ∧ X ∪ Y=X ∪ Z⇒Y=Z. Doka`i!<br />

4. Neka A i B se proiz<strong>vo</strong>lni mno`estva. Doka`i deka slednive uslovi<br />

se ekvivalentni:<br />

(i) A ⊆ B,<br />

(ii) A ∩ B=A,<br />

(iii) A ∪ B=B.<br />

5. Neka A i B se proiz<strong>vo</strong>lni mno`estva. Doka`i deka:<br />

(a) A ⊆ B⇔A ∩ B'=∅;<br />

(b) A ∪ B = ∅⇔A=∅ ∧ B=∅;<br />

(v) A=B⇔A+B=∅.<br />

6. Da se poka`e deka (A ∩ B) ∪ (C ∩ D) ⊆ (A ∪ C) ∩ (B ∪ D). Dali va`i<br />

ravenst<strong>vo</strong>?<br />

7. Da se doka`e to~nosta <strong>na</strong> ravenstvata:<br />

35

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