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Question

(Distributive laws) For any three sets A, B, C prove that:

I. A(BC)=(AB)(AC)
[Distirbutive law of union over intersection]

II. A[(BC)]=(AB)(AC)
[Distributive law of intersection over union]


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    Solution

    I. Let x be an arbitary element of A(BC). Then,

    xϵA(BC) xϵA or xϵ(BC) xϵA or (xϵB) and xϵC (xϵA or xϵB) and (xϵA or xϵC)

    [ 'or' distributes 'and']

    xε(AB) and xϵ(AC). xϵ(AB)(AC)
    A(BC)(AB)(AC) (i)

    Again. let y be an arbitary element of (AB)(AC). Then,

    yϵ(AB)(AC)yϵ(AB) and yϵ(AC)(yϵA or yϵB) and (yϵA or yϵC)yϵA or (yϵB and yϵC)

    [ 'or; distributes 'and']

    yϵA or yϵ(BC) yϵA(BC) (AB)(AC)A(BC) (ii).

    From (i) and (ii), we get A(BC)=(AB)(AC)

    II. Let x be an arbitrary element of A(BC). Then,

    xϵA(BC)xϵA and xϵ(BC)xϵA and (xϵB or xϵC)(xϵA and xϵB) or (xϵA and xϵC)

    [ 'and' distributes 'or']

    xϵ(AB) or xϵ(AC) xϵ(AB)(AC)

    A(BC)(AB)(AC) (iii)

    Again, let y be an arbitrary element of (AB)(AC). Then,

    yϵ(AB)(AC)yϵ(AB) or yϵ(AC)(yϵA and yϵB) or (yϵA and yϵC)yϵA and (yϵB or yϵC)

    [ and' distributes 'or']

    yϵA and yϵ(BC)yϵA(BC).

    (AB)(AC)A(BC) (iv)

    From (iii) and (iv), we get A(BC)=(AB)(AC).


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