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JEE Main 202311 AprilEvening Shift
Exam
JEE Main
Year
2023
Date
11 April
Shift
Evening Shift
Subject
Mathematics
Chapter
Complex Number

JEE Main 2023 · 11 April · Evening Shift · Mathematics · Complex Number

For a ∈ ℂ , let A = z ∈ ℂ: Re ( a + ¯ z ) > Im ( ¯ a + z ) and B =...

For a∈Ca \in \mathbb{C}, let A=z∈C:Re(a+z¯)>Im(a¯+z)A = { z \in \mathbb{C}: Re ( a + \overset{¯}{z} ) > Im ( \overset{¯}{a} + z ) } and B=z∈C:Re(a+z¯)<Im(a¯+z)B = { z \in \mathbb{C}: Re ( a + \overset{¯}{z} ) < Im ( \overset{¯}{a} + z ) }. Then among the two statements:
(S1)( S1 ) : If Re(a),Im(a)>0Re ( a ) , Im ( a ) > 0, then the set AA contains all the real numbers
(S2)( S2 ) : If Re(a),Im(a)<0Re ( a ) , Im ( a ) < 0, then the set BB contains all the real numbers,

Options

  1. AOnly (S2)\left(S2\right) is true
  2. Bonly (S1)\left(S1\right) is true
  3. CBoth are true
  4. DBoth are false

Correct answer

D. Both are false

Step-by-step solution

Let a=x1+iy1,a¯=x1−iy1a = {x}_{1} + i{y}_{1}, \overset{¯}{a} = {x}_{1} - i{y}_{1}

And z=x2+iy2,z¯=x2−iy2z = {x}_{2} + i{y}_{2}, \overset{¯}{z} = {x}_{2} - i{y}_{2}

Now,

a+z¯=(x1+x2)+i(y1−y2)a + \overset{¯}{z} = \left({x}_{1} + {x}_{2}\right) + i\left({y}_{1} - {y}_{2}\right)

and a¯+z=(x1+x2)+i(y2−y1)\overset{¯}{a} + z = \left({x}_{1} + {x}_{2}\right) + i\left({y}_{2} - {y}_{1}\right)

Now according to the question,

Re(a+z¯)=x1+x2Re ( a + \overset{¯}{z} ) = {x}_{1} + {x}_{2} and

Im(a¯+z)=y2−y1Im ( \overset{¯}{a} + z ) = {y}_{2} - {y}_{1}

A={z:x1+x2>y2−y1}={z:x1+y1+x2>y2}A = \left\{z: {x}_{1} + {x}_{2} > {y}_{2} - {y}_{1}\right\} = \left\{z: {x}_{1} + {y}_{1} + {x}_{2} > {y}_{2}\right\}
B={z:x1+x2<y2−y1}={z:x1+y1+x2<y2}B = \left\{z: {x}_{1} + {x}_{2} < {y}_{2} - {y}_{1}\right\} = \left\{z: {x}_{1} + {y}_{1} + {x}_{2} < {y}_{2}\right\}

If y2=0{y}_{2} = 0 and x1,y1>0{x}_{1}, {y}_{1} > 0 then

A=z:x2>−(x1+y1)A = { z: {x}_{2} > - \left({x}_{1} + {y}_{1}\right) }

A covers a part of negative real axis and therefore, does not contain whole real axis

Similarly if y2=0{y}_{2} = 0, and x1,y1<0{x}_{1}, {y}_{1} < 0, then
B=z:x2<−(x1+y1)B = { z: {x}_{2} < - \left({x}_{1} + {y}_{1}\right) }
∴∴ BB covers part of positive real axis and therefore does not cover whole real axis.

Hence, both are false.

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