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Complex Numbers question

2022 · Shift 1 · Q23
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  5. /2022 · Shift 1 · Q23

Complex Numbers question

2022 · Shift 1 · Q23

JEE AdvancedMathematicsComplex NumbersNumerical+3 / −1
Let zˉ\bar{z}zˉ denote the complex conjugate of a complex number zzz and let i=−1i=\sqrt{-1}i=−1​. In the set of complex numbers, the number of distinct roots of the equation zˉ−z2=i(zˉ+z2)\bar{z}-z^{2}=i\left(\bar{z}+z^{2}\right)zˉ−z2=i(zˉ+z2) is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 4

  1. We need to solve zˉ−z2=i(zˉ+z2).\bar z-z^2=i(\bar z+z^2).zˉ−z2=i(zˉ+z2). Let z=x+iy,zˉ=x−iy,z=x+iy, \qquad \bar z=x-iy,z=x+iy,zˉ=x−iy, where x,y∈Rx,y\in\mathbb Rx,y∈R.

  2. Rearrange the equation: zˉ−z2=izˉ+iz2\bar z-z^2=i\bar z+iz^2zˉ−z2=izˉ+iz2 zˉ−izˉ=z2+iz2\bar z-i\bar z=z^2+iz^2zˉ−izˉ=z2+iz2 (1−i)zˉ=(1+i)z2.(1-i)\bar z=(1+i)z^2.(1−i)zˉ=(1+i)z2.

Now divide by (1−i)(1-i)(1−i): zˉ=1+i1−iz2.\bar z=\frac{1+i}{1-i}z^2.zˉ=1−i1+i​z2. Since 1+i1−i=(1+i)2(1−i)(1+i)=1+2i+i22=2i2=i,\frac{1+i}{1-i}=\frac{(1+i)^2}{(1-i)(1+i)}=\frac{1+2i+i^2}{2}=\frac{2i}{2}=i,1−i1+i​=(1−i)(1+i)(1+i)2​=21+2i+i2​=22i​=i, we get zˉ=iz2.\bar z=iz^2.zˉ=iz2. So we must solve x−iy=i(x+iy)2.x-iy=i(x+iy)^2.x−iy=i(x+iy)2.

  1. Compute z2z^2z2: (x+iy)2=x2−y2+2ixy.(x+iy)^2=x^2-y^2+2ixy.(x+iy)2=x2−y2+2ixy. Hence iz2=i(x2−y2+2ixy)=i(x2−y2)−2xy.iz^2=i(x^2-y^2+2ixy)=i(x^2-y^2)-2xy.iz2=i(x2−y2+2ixy)=i(x2−y2)−2xy. So iz2=−2xy+i(x2−y2).iz^2=-2xy+i(x^2-y^2).iz2=−2xy+i(x2−y2).

Equating real and imaginary parts in x−iy=−2xy+i(x2−y2),x-iy=-2xy+i(x^2-y^2),x−iy=−2xy+i(x2−y2), we get x=−2xy...(1)x=-2xy \qquad ...(1)x=−2xy...(1) −y=x2−y2...(2)-y=x^2-y^2 \qquad ...(2)−y=x2−y2...(2)

  1. Solve the system.

From (1): x=−2xyx=-2xyx=−2xy x(1+2y)=0.x(1+2y)=0.x(1+2y)=0. So either

  • x=0x=0x=0, or
  • 1+2y=0⇒y=−121+2y=0 \Rightarrow y=-\frac121+2y=0⇒y=−21​.

Case 1: x=0x=0x=0

Then from (2): −y=0−y2=−y2-y=0-y^2=-y^2−y=0−y2=−y2 y=y2y=y^2y=y2 y(y−1)=0.y(y-1)=0.y(y−1)=0. Thus y=0ory=1.y=0 \quad \text{or} \quad y=1.y=0ory=1. So the roots are z=0,z=i.z=0, \quad z=i.z=0,z=i.


Case 2: y=−12y=-\frac12y=−21​

Substitute into (2): −(−12)=x2−(14)-\left(-\frac12\right)=x^2-\left(\frac14\right)−(−21​)=x2−(41​) 12=x2−14\frac12=x^2-\frac1421​=x2−41​ x2=34.x^2=\frac34.x2=43​. Thus x=±32.x=\pm \frac{\sqrt3}{2}.x=±23​​. So the roots are z=32−i2,z=−32−i2.z=\frac{\sqrt3}{2}-\frac{i}{2}, \quad z=-\frac{\sqrt3}{2}-\frac{i}{2}.z=23​​−2i​,z=−23​​−2i​.

  1. Therefore the distinct roots are: 0, i, 32−i2, −32−i2.0,\ i,\ \frac{\sqrt3}{2}-\frac{i}{2},\ -\frac{\sqrt3}{2}-\frac{i}{2}.0, i, 23​​−2i​, −23​​−2i​. Hence, the number of distinct roots is 4.\boxed{4}.4​.

  2. Comparison with stored answer: Stored correct answer = 444. This matches our result.

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