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JEE Main 20199 AprilEvening Shift
Exam
JEE Main
Year
2019
Date
9 April
Shift
Evening Shift
Subject
Mathematics
Chapter
Complex Number

JEE Main 2019 · 9 April · Evening Shift · Mathematics · Complex Number

Let z ∈ C be such that |z| < 1 . If ω = (5 + 3z)/(5(1 - z)), then:

Let z∈Cz \in C be such that ∣z∣<1.\left|z\right| < 1 . If ω=5+3z5(1−z),\omega = \dfrac{5 + 3z}{5\left(1 - z\right)}, then:Let z ∈ C be such that |z| < 1 . If ω = (5 + 3z)/(5(1 - z)), then:

Options

  1. A5Re(ω)>15Re\left(\omega\right) > 1
  2. B5Im(ω)<15Im\left(\omega\right) < 1
  3. C5Re(ω)>45Re\left(\omega\right) > 4
  4. D4Im(ω)>54Im\left(\omega\right) > 5

Correct answer

A. 5Re(ω)>15Re\left(\omega\right) > 1

Step-by-step solution

Given ∣z∣<1\left|z\right| < 1 and ω=5+3z5(1−z)\omega = \dfrac{5 + 3z}{5\left(1 - z\right)}

⇒5ω(1−z)=5+3z⇒ 5\omega\left(1 - z\right) = 5 + 3z

⇒5ω−5\omegaz=5+3z⇒ 5\omega - 5\omegaz = 5 + 3z

⇒z=5ω−53+5ω⇒ z = \dfrac{5\omega - 5}{3 + 5\omega}

Using the given condition \left|z\right| = 5\left|\dfrac{\omega - 1}{3 + 5\omega}\right| < 1</p><p><imgsrc="https://cdn−question−pool.getmarks.app/pyq/jeemain/6d93483a−6d23−4e29−a9d9−139fafe1e404−image8.png"style="width:250px;height:160px;"/><br/></p><p><img src="https://cdn-question-pool.Quantrex.app/pyq/jee_main/6d93483a-6d23-4e29-a9d9-139fafe1e404-image8.png" style="width: 250px; height: 160px;" /><br />⇒ 5\left|\omega - 1\right| < \left|3 + 5\omega\right|</p><p></p><p>⇒ 5\left|\omega - 1\right| < 5\left|\omega + \dfrac{3}{5}\right|</p><p></p><p>⇒ \left|\omega - 1\right| < \left|\omega - \left(- \dfrac{3}{5}\right)\right|</p><p>Weknowthatthelocusofacomplexnumber</p><p>We know that the locus of a complex number{z}_{1}satisfying∣z1−a∣=∣z2−b∣satisfying \left|{z}_{1} - a\right| = \left|{z}_{2} - b\right| is the perpendicular bisector of the line segment joining the points aa and b.b .

Hence, the locus of the point ω\omega satisfying \left|\omega - 1\right| = ∣ω−(−35)∣\left|\omega - \left(- \dfrac{3}{5}\right)\right| is the line x=1−352=15x = \dfrac{1 - \dfrac{3}{5}}{2} = \dfrac{1}{5}

Hence, Re(ω)>15Re\left(\omega\right) > \dfrac{1}{5}

⇒5Re(ω)>1.⇒ 5Re\left(\omega\right) > 1 .

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