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Complex Numbers question

2022 · Shift 2 · Q29
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  5. /2022 · Shift 2 · Q29

Complex Numbers question

2022 · Shift 2 · Q29

JEE AdvancedMathematicsComplex NumbersMultiple correct+4 / −2
Let zˉ\bar{z}zˉ denote the complex conjugate of a complex number zzz. If zzz is a non-zero complex number for which both real and imaginary parts of (zˉ)2+1z2(\bar{z})^{2}+\frac{1}{z^{2}}(zˉ)2+z21​ are integers, then which of the following is/are possible value(s) of ∣z∣|z|∣z∣ ?
  1. A
    (43+32052)14\left(\frac{43+3 \sqrt{205}}{2}\right)^{\frac{1}{4}}(243+3205​​)41​
  2. B
    (7+334)14\left(\frac{7+\sqrt{33}}{4}\right)^{\frac{1}{4}}(47+33​​)41​
  3. C
    (9+654)14\left(\frac{9+\sqrt{65}}{4}\right)^{\frac{1}{4}}(49+65​​)41​
  4. D
    (7+136)14\left(\frac{7+\sqrt{13}}{6}\right)^{\frac{1}{4}}(67+13​​)41​
View written solutionFree

Correct answer: A

Step-by-step Solution

  1. Let the given complex number be w=(zˉ)2+1z2w = (\bar{z})^{2}+\frac{1}{z^{2}}w=(zˉ)2+z21​. We are given that the real and imaginary parts of www are integers. Let Re(w)=kRe(w) = kRe(w)=k and Im(w)=mIm(w) = mIm(w)=m, where k,m∈Zk, m \in \mathbb{Z}k,m∈Z.

  2. Let's express zzz in its polar form, z=reiθz = r e^{i\theta}z=reiθ, where r=∣z∣r = |z|r=∣z∣ is the modulus and θ\thetaθ is the argument. The complex conjugate is zˉ=re−iθ\bar{z} = r e^{-i\theta}zˉ=re−iθ.

  3. Now, we substitute this into the expression for www: (zˉ)2=(re−iθ)2=r2e−i2θ(\bar{z})^2 = (r e^{-i\theta})^2 = r^2 e^{-i2\theta}(zˉ)2=(re−iθ)2=r2e−i2θ z2=(reiθ)2=r2ei2θz^2 = (r e^{i\theta})^2 = r^2 e^{i2\theta}z2=(reiθ)2=r2ei2θ 1z2=1r2ei2θ=1r2e−i2θ\frac{1}{z^2} = \frac{1}{r^2 e^{i2\theta}} = \frac{1}{r^2} e^{-i2\theta}z21​=r2ei2θ1​=r21​e−i2θ

  4. So, the expression for www becomes: w=r2e−i2θ+1r2e−i2θ=(r2+1r2)e−i2θw = r^2 e^{-i2\theta} + \frac{1}{r^2} e^{-i2\theta} = \left(r^2 + \frac{1}{r^2}\right) e^{-i2\theta}w=r2e−i2θ+r21​e−i2θ=(r2+r21​)e−i2θ

  5. Using Euler's formula, e−i2θ=cos⁡(2θ)−isin⁡(2θ)e^{-i2\theta} = \cos(2\theta) - i\sin(2\theta)e−i2θ=cos(2θ)−isin(2θ), we can write www in Cartesian form: w=(r2+1r2)(cos⁡(2θ)−isin⁡(2θ))w = \left(r^2 + \frac{1}{r^2}\right) (\cos(2\theta) - i\sin(2\theta))w=(r2+r21​)(cos(2θ)−isin(2θ))

  6. From this, we can identify the real and imaginary parts of www: Re(w)=(r2+1r2)cos⁡(2θ)=kRe(w) = \left(r^2 + \frac{1}{r^2}\right) \cos(2\theta) = kRe(w)=(r2+r21​)cos(2θ)=k (Equation 1) Im(w)=−(r2+1r2)sin⁡(2θ)=mIm(w) = -\left(r^2 + \frac{1}{r^2}\right) \sin(2\theta) = mIm(w)=−(r2+r21​)sin(2θ)=m (Equation 2)

  7. To eliminate θ\thetaθ, we can square both equations and add them. This uses the trigonometric identity cos⁡2(2θ)+sin⁡2(2θ)=1\cos^2(2\theta) + \sin^2(2\theta) = 1cos2(2θ)+sin2(2θ)=1. k2=(r2+1r2)2cos⁡2(2θ)k^2 = \left(r^2 + \frac{1}{r^2}\right)^2 \cos^2(2\theta)k2=(r2+r21​)2cos2(2θ) m2=(r2+1r2)2sin⁡2(2θ)m^2 = \left(r^2 + \frac{1}{r^2}\right)^2 \sin^2(2\theta)m2=(r2+r21​)2sin2(2θ) Adding these two gives: k2+m2=(r2+1r2)2(cos⁡2(2θ)+sin⁡2(2θ))k^2 + m^2 = \left(r^2 + \frac{1}{r^2}\right)^2 (\cos^2(2\theta) + \sin^2(2\theta))k2+m2=(r2+r21​)2(cos2(2θ)+sin2(2θ)) k2+m2=(r2+1r2)2k^2 + m^2 = \left(r^2 + \frac{1}{r^2}\right)^2k2+m2=(r2+r21​)2

  8. Let's expand the right side and express it in terms of R=∣z∣4=r4R = |z|^4 = r^4R=∣z∣4=r4. k2+m2=(r2)2+2(r2)(1r2)+(1r2)2=r4+2+1r4k^2 + m^2 = (r^2)^2 + 2(r^2)\left(\frac{1}{r^2}\right) + \left(\frac{1}{r^2}\right)^2 = r^4 + 2 + \frac{1}{r^4}k2+m2=(r2)2+2(r2)(r21​)+(r21​)2=r4+2+r41​ k2+m2=R+2+1Rk^2 + m^2 = R + 2 + \frac{1}{R}k2+m2=R+2+R1​ This gives us a necessary condition on R=∣z∣4R = |z|^4R=∣z∣4: R+1R=k2+m2−2R + \frac{1}{R} = k^2 + m^2 - 2R+R1​=k2+m2−2

  9. Since kkk and mmm are integers, k2+m2−2k^2+m^2-2k2+m2−2 must be an integer. Therefore, for a value of ∣z∣|z|∣z∣ to be possible, ∣z∣4+1∣z∣4|z|^4 + \frac{1}{|z|^4}∣z∣4+∣z∣41​ must be an integer.

  10. Now we test each option by calculating R=∣z∣4R = |z|^4R=∣z∣4 and checking if R+1RR + \frac{1}{R}R+R1​ is an integer.

    A: ∣z∣=(43+32052)14|z| = \left(\frac{43+3 \sqrt{205}}{2}\right)^{\frac{1}{4}}∣z∣=(243+3205​​)41​ R=∣z∣4=43+32052R = |z|^4 = \frac{43+3 \sqrt{205}}{2}R=∣z∣4=243+3205​​. 1R=243+3205=2(43−3205)(43)2−(3205)2=2(43−3205)1849−9(205)=2(43−3205)1849−1845=2(43−3205)4=43−32052\frac{1}{R} = \frac{2}{43+3 \sqrt{205}} = \frac{2(43-3 \sqrt{205})}{(43)^2 - (3\sqrt{205})^2} = \frac{2(43-3 \sqrt{205})}{1849 - 9(205)} = \frac{2(43-3 \sqrt{205})}{1849 - 1845} = \frac{2(43-3 \sqrt{205})}{4} = \frac{43-3 \sqrt{205}}{2}R1​=43+3205​2​=(43)2−(3205​)22(43−3205​)​=1849−9(205)2(43−3205​)​=1849−18452(43−3205​)​=42(43−3205​)​=243−3205​​. R+1R=43+32052+43−32052=862=43R + \frac{1}{R} = \frac{43+3 \sqrt{205}}{2} + \frac{43-3 \sqrt{205}}{2} = \frac{86}{2} = 43R+R1​=243+3205​​+243−3205​​=286​=43. Since 43 is an integer, this option is potentially correct. We need to check if k2+m2−2=43k^2+m^2-2 = 43k2+m2−2=43, which means k2+m2=45k^2+m^2=45k2+m2=45. The number 45 can be written as a sum of two squares, for example, 45=62+3245 = 6^2 + 3^245=62+32. So we can have integers k=6,m=3k=6, m=3k=6,m=3. Thus, this option is possible.

    B: ∣z∣=(7+334)14|z| = \left(\frac{7+\sqrt{33}}{4}\right)^{\frac{1}{4}}∣z∣=(47+33​​)41​ R=∣z∣4=7+334R = |z|^4 = \frac{7+\sqrt{33}}{4}R=∣z∣4=47+33​​. 1R=47+33=4(7−33)49−33=4(7−33)16=7−334\frac{1}{R} = \frac{4}{7+\sqrt{33}} = \frac{4(7-\sqrt{33})}{49 - 33} = \frac{4(7-\sqrt{33})}{16} = \frac{7-\sqrt{33}}{4}R1​=7+33​4​=49−334(7−33​)​=164(7−33​)​=47−33​​. R+1R=7+334+7−334=144=72R + \frac{1}{R} = \frac{7+\sqrt{33}}{4} + \frac{7-\sqrt{33}}{4} = \frac{14}{4} = \frac{7}{2}R+R1​=47+33​​+47−33​​=414​=27​. This is not an integer. Therefore, this option is not possible.

    C: ∣z∣=(9+654)14|z| = \left(\frac{9+\sqrt{65}}{4}\right)^{\frac{1}{4}}∣z∣=(49+65​​)41​ R=∣z∣4=9+654R = |z|^4 = \frac{9+\sqrt{65}}{4}R=∣z∣4=49+65​​. 1R=49+65=4(9−65)81−65=4(9−65)16=9−654\frac{1}{R} = \frac{4}{9+\sqrt{65}} = \frac{4(9-\sqrt{65})}{81 - 65} = \frac{4(9-\sqrt{65})}{16} = \frac{9-\sqrt{65}}{4}R1​=9+65​4​=81−654(9−65​)​=164(9−65​)​=49−65​​. R+1R=9+654+9−654=184=92R + \frac{1}{R} = \frac{9+\sqrt{65}}{4} + \frac{9-\sqrt{65}}{4} = \frac{18}{4} = \frac{9}{2}R+R1​=49+65​​+49−65​​=418​=29​. This is not an integer. Therefore, this option is not possible.

    D: ∣z∣=(7+136)14|z| = \left(\frac{7+\sqrt{13}}{6}\right)^{\frac{1}{4}}∣z∣=(67+13​​)41​ R=∣z∣4=7+136R = |z|^4 = \frac{7+\sqrt{13}}{6}R=∣z∣4=67+13​​. 1R=67+13=6(7−13)49−13=6(7−13)36=7−136\frac{1}{R} = \frac{6}{7+\sqrt{13}} = \frac{6(7-\sqrt{13})}{49 - 13} = \frac{6(7-\sqrt{13})}{36} = \frac{7-\sqrt{13}}{6}R1​=7+13​6​=49−136(7−13​)​=366(7−13​)​=67−13​​. R+1R=7+136+7−136=146=73R + \frac{1}{R} = \frac{7+\sqrt{13}}{6} + \frac{7-\sqrt{13}}{6} = \frac{14}{6} = \frac{7}{3}R+R1​=67+13​​+67−13​​=614​=37​. This is not an integer. Therefore, this option is not possible.

  11. Only option A satisfies the necessary condition. Therefore, it is the only possible value for ∣z∣|z|∣z∣ among the given choices.

Final Answer is A.

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