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JEE Main 202125 FebruaryMorning Shift
Exam
JEE Main
Year
2021
Date
25 February
Shift
Morning Shift
Subject
Mathematics
Chapter
Complex Number

JEE Main 2021 · 25 February · Morning Shift · Mathematics · Complex Number

Let the lines (2 - i)z = (2 + i) ¯ z and (2 + i)z + (i - 2) ¯ z -...

Let the lines (2−i)z=(2+i)z¯and(2+i)z+(i−2)z¯−4i=0,\left(2 - i\right)z = \left(2 + i\right)\overset{¯}{z} and \left(2 + i\right)z + \left(i - 2\right)\overset{¯}{z} - 4i = 0, (here i2=−1{i}^{2} = - 1) be normal to a circle CC. If the line iz+z¯+1+i=0iz + \overset{¯}{z} + 1 + i = 0 is tangent to this circle CC, then its radius is :

Options

  1. A32\dfrac{3}{\sqrt{2}}
  2. B323\sqrt{2}
  3. C322\dfrac{3}{2\sqrt{2}}
  4. D122\dfrac{1}{2\sqrt{2}}

Correct answer

C. 322\dfrac{3}{2\sqrt{2}}

Step-by-step solution

(2−i)z=(2+i)z¯( 2 - i ) z = ( 2 + i ) \overset{¯}{z}

⇒(2−i)(x+iy)=(2+i)(x−iy)⇒ ( 2 - i ) ( x + iy ) = ( 2 + i ) ( x - iy )

⇒2x−ix+2iy+y=2x+ix−2iy+y⇒ 2x - ix + 2iy + y = 2x + ix - 2iy + y

⇒2ix−4iy=0⇒ 2ix - 4iy = 0

L1:x−2y=0{L}_{1}: x - 2y = 0

⇒(2+i)z+(i−2)z¯−4i=0⇒ ( 2 + i ) z + ( i - 2 ) \overset{¯}{z} - 4i = 0

⇒(2+i)(x+iy)+(i−2)(x−iy)−4i=0⇒ ( 2 + i ) ( x + iy ) + ( i - 2 ) ( x - iy ) - 4i = 0

.⇒2x+ix+2iy−y+ix−2x+y+2iy−4i=0⇒ 2x + ix + 2iy - y + ix - 2x + y + 2iy - 4i = 0

⇒2ix+4iy−4i=0⇒ 2ix + 4iy - 4i = 0

Solve L1{L}_{1} and L2,4y=2,y=12{L}_{2}, 4y = 2, y = \dfrac{1}{2}

∴x=1∴ x = 1

Centre \left(1, \dfrac{1}{2}\right)</p><p></p><p>{L}_{3}: iz + \overset{¯}{z} + 1 + i = 0</p><p></p><p>⇒ i ( x + iy ) + x - iy + 1 + i = 0</p><p></p><p>⇒ ix - y + x - iy + 1 + i = 0</p><p></p><p>⇒ ( x - y + 1 ) + i ( x - y + 1 ) = 0</p><p>Radius=distancefrom(1,12)</p><p>Radius = distance from \left(1, \dfrac{1}{2}\right) to x−y+1=0x - y + 1 = 0

r=∣1−12+12∣r = \left|\dfrac{1 - \dfrac{1}{2} + 1}{\sqrt{2}}\right|

r=322r = \dfrac{3}{2\sqrt{2}}

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