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Complex Numbers question

2023 · 11 Apr · Shift 2 · Q26
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Complex Numbers question

2023 · 11 Apr · Shift 2 · Q26

JEE MainMathematicsComplex NumbersMCQ+4 / −1
For a∈Ca \in \mathbb{C}a∈C, let A={z∈C:Re⁡(a+zˉ)>Im⁡(aˉ+z)}\mathrm{A}=\{z \in \mathbb{C}: \operatorname{Re}(a+\bar{z}) \gt \operatorname{Im}(\bar{a}+z)\}A={z∈C:Re(a+zˉ)>Im(aˉ+z)} and B={z∈C:Re⁡(a+zˉ)(S1):If\mathrm{B}=\{z \in \mathbb{C}: \operatorname{Re}(a+\bar{z})(S1): IfB={z∈C:Re(a+zˉ)(S1):If\operatorname{Re}(a), \operatorname{Im}(a) > 0 ,thenthesetAcontainsalltherealnumbers(S2):If, then the set A contains all the real numbers (S2) : If,thenthesetAcontainsalltherealnumbers(S2):If\operatorname{Re}(a), \operatorname{Im}(a) < 0$$, then the set B contains all the real numbers,
  1. A
    both are false
  2. B
    only (S1) is true
  3. C
    only (S2) is true
  4. D
    both are true
View written solutionFree

Correct answer: A

Let a=α+iβ,z=x+iya=\alpha+i\beta,\qquad z=x+iya=α+iβ,z=x+iy where α=Re⁡(a)\alpha=\operatorname{Re}(a)α=Re(a), β=Im⁡(a)\beta=\operatorname{Im}(a)β=Im(a) and x,y∈Rx,y\in\mathbb Rx,y∈R.

We will simplify the conditions defining AAA and BBB.


1. Simplify set AAA

We have zˉ=x−iy,aˉ=α−iβ.\bar z=x-iy,\qquad \bar a=\alpha-i\beta.zˉ=x−iy,aˉ=α−iβ. Then a+zˉ=(α+iβ)+(x−iy)=(α+x)+i(β−y),a+\bar z=(\alpha+i\beta)+(x-iy)=(\alpha+x)+i(\beta-y),a+zˉ=(α+iβ)+(x−iy)=(α+x)+i(β−y), so Re⁡(a+zˉ)=α+x.\operatorname{Re}(a+\bar z)=\alpha+x.Re(a+zˉ)=α+x.

Also, aˉ+z=(α−iβ)+(x+iy)=(α+x)+i(y−β),\bar a+z=(\alpha-i\beta)+(x+iy)=(\alpha+x)+i(y-\beta),aˉ+z=(α−iβ)+(x+iy)=(α+x)+i(y−β), so Im⁡(aˉ+z)=y−β.\operatorname{Im}(\bar a+z)=y-\beta.Im(aˉ+z)=y−β.

Thus the condition for AAA is α+x>y−β\alpha+x>y-\betaα+x>y−β which gives x−y+α+β>0.x-y+\alpha+\beta>0.x−y+α+β>0. Hence A={z=x+iy:x−y+α+β>0}.A=\{z=x+iy: x-y+\alpha+\beta>0\}.A={z=x+iy:x−y+α+β>0}.


2. Simplify set BBB

The statement for BBB is truncated in the prompt, but from the visible part it begins exactly the same way: B={z∈C:Re⁡(a+zˉ)(⋯ )B=\{z\in\mathbb C: \operatorname{Re}(a+\bar z)(\cdots)B={z∈C:Re(a+zˉ)(⋯) The natural complementary comparison here is Re⁡(a+zˉ)<Im⁡(aˉ+z),\operatorname{Re}(a+\bar z)<\operatorname{Im}(\bar a+z),Re(a+zˉ)<Im(aˉ+z), so let us evaluate that.

Then α+x<y−β\alpha+x<y-\betaα+x<y−β which gives x−y+α+β<0.x-y+\alpha+\beta<0.x−y+α+β<0. Hence B={z=x+iy:x−y+α+β<0}.B=\{z=x+iy: x-y+\alpha+\beta<0\}.B={z=x+iy:x−y+α+β<0}.


3. Check statement (S1)

If Re⁡(a),Im⁡(a)>0\operatorname{Re}(a),\operatorname{Im}(a)>0Re(a),Im(a)>0, then the set AAA contains all the real numbers.

A real number z=t∈Rz=t\in\mathbb Rz=t∈R means x=t,y=0.x=t,\quad y=0.x=t,y=0. So for real z=tz=tz=t, the condition for AAA becomes t+α+β>0.t+\alpha+\beta>0.t+α+β>0.

For AAA to contain all real numbers, this inequality must hold for every t∈Rt\in\mathbb Rt∈R. But that is impossible, because for sufficiently large negative ttt, we get t+α+β<0.t+\alpha+\beta<0.t+α+β<0. For example, choose t=−(α+β+1).t=-(\alpha+\beta+1).t=−(α+β+1). Then t+α+β=−1<0.t+\alpha+\beta=-1<0.t+α+β=−1<0. So this real number is not in AAA.

Therefore, (S1) is false.


4. Check statement (S2)

If Re⁡(a),Im⁡(a)<0\operatorname{Re}(a),\operatorname{Im}(a)<0Re(a),Im(a)<0, then the set BBB contains all the real numbers.

Again let z=t∈Rz=t\in\mathbb Rz=t∈R. Then the condition for BBB becomes t+α+β<0.t+\alpha+\beta<0.t+α+β<0.

For BBB to contain all real numbers, this must hold for every real ttt. But this is impossible, because for sufficiently large positive ttt, t+α+β>0.t+\alpha+\beta>0.t+α+β>0. For example, choose t=−(α+β)+1.t=-(\alpha+\beta)+1.t=−(α+β)+1. Then t+α+β=1>0,t+\alpha+\beta=1>0,t+α+β=1>0, so this real number is not in BBB.

Therefore, (S2) is false.


5. Conclusion

Both statements are false.

So the correct option is A\boxed{A}A​


6. Comparison with stored answer

Stored correct answer: AAA

Our derived answer also gives AAA, so they agree.

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