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Complex Numbers question

2023 · Shift 1 · Q34
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  5. /2023 · Shift 1 · Q34

Complex Numbers question

2023 · Shift 1 · Q34

JEE AdvancedMathematicsComplex NumbersMCQ+3 / −1
Let zzz be a complex number satisfying ∣z∣3+2z2+4zˉ−8=0|z|^3+2 z^2+4 \bar{z}-8=0∣z∣3+2z2+4zˉ−8=0, where zˉ\bar{z}zˉ denotes the complex conjugate of zzz. Let the imaginary part of zzz be nonzero.

Match each entry in List-I to the correct entries in List-II.

List - I List - II
(P) ∣z∣2|z|^2∣z∣2 is equal to (1) 12
(Q) ∣z−zˉ∣2|z-\bar{z}|^2∣z−zˉ∣2 is equal to (2) 4
(R) ∣z∣2+∣z+zˉ∣2|z|^2+|z+\bar{z}|^2∣z∣2+∣z+zˉ∣2 is equal to (3) 8
(S) ∣z+1∣2|z+1|^2∣z+1∣2 is equal to (4) 10
(5) 7

The correct option is:
  1. A
    (P)→(1)(Q)→(3)(R)→(5)(S)→(4)(P) \rightarrow(1) \quad(Q) \rightarrow(3) \quad(R) \rightarrow(5) \quad(S) \rightarrow(4)(P)→(1)(Q)→(3)(R)→(5)(S)→(4)
  2. B
    (P)→(2)(Q)→(1)(R)→(3)(S)→(5)(P) \rightarrow(2) \quad(Q) \rightarrow(1) \quad(R) \rightarrow(3) \quad(S) \rightarrow(5)(P)→(2)(Q)→(1)(R)→(3)(S)→(5)
  3. C
    (P)→(2)(Q)→(4)(R)→(5)(S)→(1)(P) \rightarrow(2) \quad(Q) \rightarrow(4) \quad(R) \rightarrow(5) \quad(S) \rightarrow(1)(P)→(2)(Q)→(4)(R)→(5)(S)→(1)
  4. D
    (P)→(2)(Q)→(3)(R)→(5)(S)→(4)(P) \rightarrow(2) \quad(Q) \rightarrow(3) \quad(R) \rightarrow(5) \quad(S) \rightarrow(4)(P)→(2)(Q)→(3)(R)→(5)(S)→(4)
View written solutionFree

Correct answer: B

  1. Let

    \quad \bar z=x-iy, \quad |z|^2=x^2+y^2$$ where the imaginary part is nonzero, so $y\neq 0$.
  2. Given equation ∣z∣3+2z2+4zˉ−8=0|z|^3+2z^2+4\bar z-8=0∣z∣3+2z2+4zˉ−8=0 Substitute z=x+iyz=x+iyz=x+iy.

    First compute: ∣z∣3=(x2+y2)3/2|z|^3=(x^2+y^2)^{3/2}∣z∣3=(x2+y2)3/2 z2=(x+iy)2=x2−y2+2ixyz^2=(x+iy)^2=x^2-y^2+2ixyz2=(x+iy)2=x2−y2+2ixy zˉ=x−iy\bar z=x-iyzˉ=x−iy

    Hence, ∣z∣3+2(x2−y2+2ixy)+4(x−iy)−8=0|z|^3+2(x^2-y^2+2ixy)+4(x-iy)-8=0∣z∣3+2(x2−y2+2ixy)+4(x−iy)−8=0

    Separate real and imaginary parts:

    • Real part: ∣z∣3+2(x2−y2)+4x−8=0|z|^3+2(x^2-y^2)+4x-8=0∣z∣3+2(x2−y2)+4x−8=0
    • Imaginary part: 4xy−4y=04xy-4y=04xy−4y=0 4y(x−1)=04y(x-1)=04y(x−1)=0

    Since y≠0y\neq 0y=0, we must have x=1x=1x=1

  3. Use x=1x=1x=1 in the real part Let r=∣z∣=x2+y2=1+y2r=|z|=\sqrt{x^2+y^2}=\sqrt{1+y^2}r=∣z∣=x2+y2​=1+y2​ Then the real part becomes r3+2(1−y2)+4−8=0r^3+2(1-y^2)+4-8=0r3+2(1−y2)+4−8=0 r3−2y2−2=0r^3-2y^2-2=0r3−2y2−2=0

    Since r2=1+y2r^2=1+y^2r2=1+y2, we have y2=r2−1y^2=r^2-1y2=r2−1 So, r3−2(r2−1)−2=0r^3-2(r^2-1)-2=0r3−2(r2−1)−2=0 r3−2r2=0r^3-2r^2=0r3−2r2=0 r2(r−2)=0r^2(r-2)=0r2(r−2)=0

    As r=∣z∣>0r=|z|>0r=∣z∣>0, we get r=2r=2r=2 Therefore, ∣z∣2=r2=4|z|^2=r^2=4∣z∣2=r2=4

  4. Find yyy x2+y2=4x^2+y^2=4x2+y2=4 1+y2=41+y^2=41+y2=4 y2=3y^2=3y2=3 Since y≠0y\neq 0y=0, this is valid.

  5. Now evaluate each quantity

    (P) ∣z∣2|z|^2∣z∣2

    ∣z∣2=4|z|^2=4∣z∣2=4 So, P→(2)P\to (2)P→(2)

    (Q) ∣z−zˉ∣2|z-\bar z|^2∣z−zˉ∣2

    z−zˉ=(x+iy)−(x−iy)=2iyz-\bar z=(x+iy)-(x-iy)=2iyz−zˉ=(x+iy)−(x−iy)=2iy ∣z−zˉ∣2=∣2iy∣2=4y2=4⋅3=12|z-\bar z|^2=|2iy|^2=4y^2=4\cdot 3=12∣z−zˉ∣2=∣2iy∣2=4y2=4⋅3=12 So, Q→(1)Q\to (1)Q→(1)

    (R) ∣z∣2+∣z+zˉ∣2|z|^2+|z+\bar z|^2∣z∣2+∣z+zˉ∣2

    z+zˉ=2x=2z+\bar z=2x=2z+zˉ=2x=2 ∣z+zˉ∣2=∣2∣2=4|z+\bar z|^2=|2|^2=4∣z+zˉ∣2=∣2∣2=4 Hence, ∣z∣2+∣z+zˉ∣2=4+4=8|z|^2+|z+\bar z|^2=4+4=8∣z∣2+∣z+zˉ∣2=4+4=8 So, R→(3)R\to (3)R→(3)

    (S) ∣z+1∣2|z+1|^2∣z+1∣2

    z+1=(x+1)+iy=2+iyz+1=(x+1)+iy=2+iyz+1=(x+1)+iy=2+iy ∣z+1∣2=22+y2=4+3=7|z+1|^2=2^2+y^2=4+3=7∣z+1∣2=22+y2=4+3=7 So, S→(5)S\to (5)S→(5)

  6. Final matching P→(2),Q→(1),R→(3),S→(5)P\to (2),\quad Q\to (1),\quad R\to (3),\quad S\to (5)P→(2),Q→(1),R→(3),S→(5)

    This corresponds to Option B.

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