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Prove or disprove: if A is a subset of B and B is not a subset of C, then A is not a subset of C.

I know it is false for the counter example:

A = {1, 2}

B = {1, 2, 3, 4}

C = {1, 2, 6, 5}

How can I prove that mathematically?

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    All you need to do to prove that something is false is to provide a counterexample. It seems you've done that.2012-03-20

4 Answers 4

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You just did; it's called a Counterexample.

You simply note that these particular $A$, $B$, and $C$ satisfy the hypotheses ("$A$ is a subset of $B$ and $B$ is not a subset of $C$"), but fail to satisfy the proposed conclusion ("$A$ is not a subset of $C$"). So the proposed implication cannot always hold. This disproves the assertion.

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There exist an element x that is a member of set A and since set A is a subset of set B, then that element x is also a member of set B.

Since set B is not a subset of set C that there does not exist an element x which is a member of set C, therefore element x is not in set C

Therefore element x is a member of set A but not an element in set C therefore set A is not a subset of set C.

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A is the subset of B given: B is not the subset of C given: Let x be an arbitrary element such that x belongs to A than we have to show that x doesn't belong to C =>xEA (:. given) =>xEB (A is subset of B) =>x doesn't belong to C (B is not the subset of C) but x is an arbitraray element of A hence proved that A is not the subset of C

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    You correctly proved that "If $A$ is a subset of $B$, and $B$ is **disjoint** from $C$, then $A$ is disjoint from $C$." However, the problem is different.2012-05-17
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You can say that if $A$ is included in $B \cap C$ then it is obviously a subset of $B$ and $C$ by definition of the intersection. But $B$ need not to be a subset of $C$ for $B \cap C$ to be non empty hence for $A$ to be non empty. Your example illustrate this quite well