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Sets and Relations question

2019 · 12 Apr · Shift 2 · Q23
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  5. /2019 · 12 Apr · Shift 2 · Q23

Sets and Relations question

2019 · 12 Apr · Shift 2 · Q23

JEE MainMathematicsSets and RelationsMCQ+4 / −1
Let A, B and C be sets such that ϕe\phi eϕe A ∩\cap∩ B ⊆\subseteq⊆ C. Then which of the following statements is not true ?
  1. A
    If (A – B) ⊆\subseteq⊆ C, then A ⊆\subseteq⊆ C
  2. B
    B ∩\cap∩ C eϕe \phieϕ
  3. C
    (C ∪\cup∪ A) ∩\cap∩ (C ∪\cup∪ B) = C
  4. D
    If (A – C) ⊆\subseteq⊆ B, then A ⊆\subseteq⊆ B
View written solutionFree

Correct answer: D

We are given A∩B⊆CA \cap B \subseteq CA∩B⊆C and we must find which statement is not true.


1. Interpret the condition

The given condition means:

  • every element common to both AAA and BBB must belong to CCC.

Equivalently, A∩B⊆C.A \cap B \subseteq C.A∩B⊆C.

We now test each option.


2. Option A

Statement: If (A−B)⊆C(A-B) \subseteq C(A−B)⊆C, then A⊆CA \subseteq CA⊆C.

Recall A=(A−B)∪(A∩B).A=(A-B)\cup(A\cap B).A=(A−B)∪(A∩B).

Given:

  • (A−B)⊆C(A-B)\subseteq C(A−B)⊆C
  • and from the question, A∩B⊆CA\cap B\subseteq CA∩B⊆C

Therefore both parts of AAA are contained in CCC. Hence A=(A−B)∪(A∩B)⊆C.A=(A-B)\cup(A\cap B)\subseteq C.A=(A−B)∪(A∩B)⊆C.

So Option A is true.


3. Option B

The printed option appears as: B∩C eϕB\cap C\ e\phiB∩C eϕ which is clearly a typographical issue. In such set notation, this is intended to mean B∩C≠ϕ.B\cap C\neq \phi.B∩C=ϕ.

Now check whether this must be true from A∩B⊆CA\cap B\subseteq CA∩B⊆C.

It need not be true.

Take a counterexample:

  • A={1}A=\{1\}A={1}
  • B={1}B=\{1\}B={1}
  • C={1}C=\{1\}C={1}

Then A∩B={1}⊆CA\cap B=\{1\}\subseteq CA∩B={1}⊆C, but here B∩C={1}≠ϕ,B\cap C=\{1\}\neq \phi,B∩C={1}=ϕ, so this example does not disprove it.

Try another:

  • A=ϕA=\phiA=ϕ
  • B={1}B=\{1\}B={1}
  • C=ϕC=\phiC=ϕ

Then A∩B=ϕ⊆C,A\cap B=\phi\subseteq C,A∩B=ϕ⊆C, which satisfies the condition. But B∩C={1}∩ϕ=ϕ.B\cap C=\{1\}\cap \phi=\phi.B∩C={1}∩ϕ=ϕ. So the statement B∩C≠ϕB\cap C\neq \phiB∩C=ϕ is false in this case.

Hence Option B is not always true.


4. Option C

Statement: (C∪A)∩(C∪B)=C(C\cup A)\cap(C\cup B)=C(C∪A)∩(C∪B)=C

Use the distributive identity: (X∪Y)∩(X∪Z)=X∪(Y∩Z).(X\cup Y)\cap(X\cup Z)=X\cup(Y\cap Z).(X∪Y)∩(X∪Z)=X∪(Y∩Z).

So, (C∪A)∩(C∪B)=C∪(A∩B).(C\cup A)\cap(C\cup B)=C\cup(A\cap B).(C∪A)∩(C∪B)=C∪(A∩B).

Given A∩B⊆CA\cap B\subseteq CA∩B⊆C, we get C∪(A∩B)=C.C\cup(A\cap B)=C.C∪(A∩B)=C.

Therefore, (C∪A)∩(C∪B)=C.(C\cup A)\cap(C\cup B)=C.(C∪A)∩(C∪B)=C.

So Option C is true.


5. Option D

Statement: If (A−C)⊆B(A-C)\subseteq B(A−C)⊆B, then A⊆BA\subseteq BA⊆B.

Now, A=(A−C)∪(A∩C).A=(A-C)\cup(A\cap C).A=(A−C)∪(A∩C).

From the given condition A∩B⊆CA\cap B\subseteq CA∩B⊆C, we cannot conclude that A∩C⊆BA\cap C\subseteq BA∩C⊆B. So even if (A−C)⊆B(A-C)\subseteq B(A−C)⊆B, it does not force all of AAA to lie in BBB.

Let us find a counterexample.

Take:

  • A={1}A=\{1\}A={1}
  • B=ϕB=\phiB=ϕ
  • C={1}C=\{1\}C={1}

Then A∩B=ϕ⊆C,A\cap B=\phi\subseteq C,A∩B=ϕ⊆C, so the given condition holds.

Also, A−C={1}−{1}=ϕ⊆B.A-C=\{1\}-\{1\}=\phi\subseteq B.A−C={1}−{1}=ϕ⊆B.

But A={1}⊈ϕ=B.A=\{1\}\not\subseteq \phi=B.A={1}⊆ϕ=B.

Thus the implication in Option D is false.

So Option D is not true.


6. Conclusion

From the valid interpretation of the options:

  • A is true
  • C is true
  • D is false

Option B, as printed, is malformed. If interpreted as B∩C≠ϕB\cap C\neq\phiB∩C=ϕ, that statement is also not always true. However, in standard versions of this question and from the stored answer, the intended uniquely false statement is D.

Therefore the correct answer is D\boxed{D}D​

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