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Sets and Relations question

2020 · 9 Jan · Shift 2 · Q30
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  5. /2020 · 9 Jan · Shift 2 · Q30

Sets and Relations question

2020 · 9 Jan · Shift 2 · Q30

JEE MainMathematicsSets and RelationsMCQ+4 / −1
If A = {x ∈\in∈ R : |x| < 2} and B = {x ∈\in∈ R : |x – 2| ≥\ge≥ 3}; then :
  1. A
    A – B = [–1, 2)
  2. B
    A ∪\cup∪ B = R – (2, 5)
  3. C
    A ∩\cap∩ B = (–2, –1)
  4. D
    B – A = R – (–2, 5)
View written solutionFree

Correct answer: D

  1. Find the sets explicitly

Given A={x∈R:∣x∣<2}A=\{x\in \mathbb R: |x|<2\}A={x∈R:∣x∣<2} so A=(−2,2).A=(-2,2).A=(−2,2).

Now B={x∈R:∣x−2∣≥3}.B=\{x\in \mathbb R: |x-2|\ge 3\}.B={x∈R:∣x−2∣≥3}.

Solve: ∣x−2∣≥3  ⟺  x−2≥3 or x−2≤−3|x-2|\ge 3 \iff x-2\ge 3 \text{ or } x-2\le -3∣x−2∣≥3⟺x−2≥3 or x−2≤−3   ⟺  x≥5 or x≤−1.\iff x\ge 5 \text{ or } x\le -1.⟺x≥5 or x≤−1. Hence B=(−∞,−1]∪[5,∞).B=(-\infty,-1]\cup [5,\infty).B=(−∞,−1]∪[5,∞).


  1. Check each option

Option A: A−B=[−1,2)A-B=[-1,2)A−B=[−1,2)

We need elements in AAA but not in BBB.

Since A=(−2,2),B=(−∞,−1]∪[5,∞),A=(-2,2),\qquad B=(-\infty,-1]\cup [5,\infty),A=(−2,2),B=(−∞,−1]∪[5,∞), within AAA, the part belonging to BBB is only (−2,2)∩(−∞,−1]=(−2,−1].(-2,2)\cap (-\infty,-1]=(-2,-1].(−2,2)∩(−∞,−1]=(−2,−1]. Therefore A−B=(−2,2)∖(−2,−1]=(−1,2).A-B=(-2,2)\setminus (-2,-1]=( -1,2).A−B=(−2,2)∖(−2,−1]=(−1,2).

But option A says [−1,2),[-1,2),[−1,2), which includes −1-1−1. Since −1∈B-1\in B−1∈B, it cannot be in A−BA-BA−B.

So A is false.


Option B: A∪B=R−(2,5)A\cup B=\mathbb R-(2,5)A∪B=R−(2,5)

Compute: A∪B=(−2,2)∪(−∞,−1]∪[5,∞).A\cup B = (-2,2)\cup (-\infty,-1]\cup [5,\infty).A∪B=(−2,2)∪(−∞,−1]∪[5,∞). Now (−∞,−1]∪(−2,2)=(−∞,2).(-\infty,-1]\cup (-2,2)=(-\infty,2).(−∞,−1]∪(−2,2)=(−∞,2). So A∪B=(−∞,2)∪[5,∞)=R−[2,5).A\cup B = (-\infty,2)\cup [5,\infty)=\mathbb R-[2,5).A∪B=(−∞,2)∪[5,∞)=R−[2,5).

But option B says R−(2,5)=(−∞,2]∪[5,∞),\mathbb R-(2,5)=(-\infty,2]\cup [5,\infty),R−(2,5)=(−∞,2]∪[5,∞), which contains 222. Since 2∉A2\notin A2∈/A and 2∉B2\notin B2∈/B, it is not in A∪BA\cup BA∪B.

So B is false.


Option C: A∩B=(−2,−1)A\cap B=(-2,-1)A∩B=(−2,−1)

Compute intersection: A∩B=(−2,2)∩[(−∞,−1]∪[5,∞)].A\cap B = (-2,2)\cap \left[(-\infty,-1]\cup [5,\infty)\right].A∩B=(−2,2)∩[(−∞,−1]∪[5,∞)]. The second part [5,∞)[5,\infty)[5,∞) has no overlap with (−2,2)(-2,2)(−2,2), so A∩B=(−2,2)∩(−∞,−1]=(−2,−1].A\cap B = (-2,2)\cap (-\infty,-1]=(-2,-1].A∩B=(−2,2)∩(−∞,−1]=(−2,−1].

Option C gives (−2,−1)(-2,-1)(−2,−1), which excludes −1-1−1. But −1∈A-1\in A−1∈A and also ∣−1−2∣=3|-1-2|=3∣−1−2∣=3, so −1∈B-1\in B−1∈B. Hence −1∈A∩B-1\in A\cap B−1∈A∩B.

So C is false.


Option D: B−A=R−(−2,5)B-A=\mathbb R-(-2,5)B−A=R−(−2,5)

Compute B−AB-AB−A, i.e. elements of BBB not in AAA.

We have B=(−∞,−1]∪[5,∞),A=(−2,2).B=(-\infty,-1]\cup [5,\infty), \qquad A=(-2,2).B=(−∞,−1]∪[5,∞),A=(−2,2). Removing AAA from BBB:

  • From (−∞,−1](-\infty,-1](−∞,−1], remove (−2,2)(-2,2)(−2,2), leaving (−∞,−2](-\infty,-2](−∞,−2].
  • The set [5,∞)[5,\infty)[5,∞) is unaffected.

Thus B−A=(−∞,−2]∪[5,∞).B-A=(-\infty,-2]\cup [5,\infty).B−A=(−∞,−2]∪[5,∞). Now R−(−2,5)=(−∞,−2]∪[5,∞).\mathbb R-(-2,5)=(-\infty,-2]\cup [5,\infty).R−(−2,5)=(−∞,−2]∪[5,∞). These are equal.

So D is true.


  1. Final answer

The only correct option is D.\boxed{D}.D​.

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