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JEE Main 202312 AprilMorning Shift
Exam
JEE Main
Year
2023
Date
12 April
Shift
Morning Shift
Subject
Mathematics
Chapter
Complex Number

JEE Main 2023 · 12 April · Morning Shift · Mathematics · Complex Number

Let C be the circle in the complex plane with centre z _ 0 =...

Let CC be the circle in the complex plane with centre z0=12(1+3i)andradius{z}_{0} = \dfrac{1}{2}\left(1 + 3i\right) and radiusr = 1.Let. Let{z}_{1} = 1 + iandthecomplexnumberand the complex number{z}_{2}beoutsidecirclebe outside circleCsuchthat∣z1−z0∣∣z2−z0∣=1such that \left|{z}_{1} - {z}_{0}\right|\left|{z}_{2} - {z}_{0}\right| = 1. If z0,z1andz2{z}_{0}, {z}_{1} and {z}_{2} are collinear, then the smaller value of ∣z2∣{\left|{z}_{2}\right|}^{2}isequaltois equal to

Options

  1. A52\dfrac{5}{2}
  2. B72\dfrac{7}{2}
  3. C132\dfrac{13}{2}
  4. D32\dfrac{3}{2}

Correct answer

A. 52\dfrac{5}{2}

Step-by-step solution

Given,

z0=1+3i2,z1=(1+i){z}_{0} = \dfrac{1 + 3i}{2}, {z}_{1} = \left(1 + i\right)

So, \left|{z}_{1} - {z}_{0}\right| = \sqrt{{\left(1 - \dfrac{1}{2}\right)}^{2} + {\left(1 - \dfrac{3}{2}\right)}^{2}} = \sqrt{\dfrac{1}{4} + \dfrac{1}{4}} = \dfrac{1}{\sqrt{2}}</p><p>And</p><p>And{z}_{2}beoutsidecircle,∣z1−z0∣∣z2−z0∣=1be outside circle, \left|{z}_{1} - {z}_{0}\right|\left|{z}_{2} - {z}_{0}\right| = 1

⇒12∣z2−z0∣=1⇒ \dfrac{1}{\sqrt{2}}\left|{z}_{2} - {z}_{0}\right| = 1

⇒∣z2−z0∣=2⇒ \left|{z}_{2} - {z}_{0}\right| = \sqrt{2}

Now given, z0,z1 and z2{z}_{0}, {z}_{1} \text{ and } {z}_{2} are collinear,

So, by concept of rotation we get,

z2−z0z1−z0=∣z2−z0∣∣z1−z0∣ei0\dfrac{{z}_{2} - {z}_{0}}{{z}_{1} - {z}_{0}} = \dfrac{\left|{z}_{2} - {z}_{0}\right|}{\left|{z}_{1} - {z}_{0}\right|}{e}^{i0}

⇒z2−z0z1−z0=∣z2−z0∣∣z1−z0∣(±1)⇒ \dfrac{{z}_{2} - {z}_{0}}{{z}_{1} - {z}_{0}} = \dfrac{\left|{z}_{2} - {z}_{0}\right|}{\left|{z}_{1} - {z}_{0}\right|}\left(\pm 1\right)

⇒z2−z0z1−z0=±2⇒ \dfrac{{z}_{2} - {z}_{0}}{{z}_{1} - {z}_{0}} = \pm 2

⇒z2=z0±2(z1−z0)⇒ {z}_{2} = {z}_{0} \pm 2\left({z}_{1} - {z}_{0}\right)

So, z2=2z1−z0=32+12i⇒∣z2∣2=52{z}_{2} = 2{z}_{1} - {z}_{0} = \dfrac{3}{2} + \dfrac{1}{2}i ⇒ {\left|{z}_{2}\right|}^{2} = \dfrac{5}{2}

Or z2=3z0−2z1=−12+52i⇒∣z2∣2=132{z}_{2} = 3{z}_{0} - 2{z}_{1} = \dfrac{- 1}{2} + \dfrac{5}{2}i ⇒ {\left|{z}_{2}\right|}^{2} = \dfrac{13}{2}

Hence, the smaller value will be, ∣z2∣2=52{\left|{z}_{2}\right|}^{2} = \dfrac{5}{2}

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