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Complex Numbers question

2008 · Shift 1 · Q24
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Complex Numbers question

2008 · Shift 1 · Q24

JEE AdvancedMathematicsComplex NumbersMCQ+3 / −1
Let A, B, C be three sets of complex numbers as defined below : A={z: Imolimits  z  ≥ 1}B={z:  ∣z−2−i∣=3}C={z: Reolimits(1−i)z)=2 }A = \left\{ {z:\,{\mathop{\rm Im} olimits} \,\,z\,\, \ge \,1} \right\}B = \left\{ {z:\,\,\left| {z - 2 - i} \right| = 3} \right\}C = \left\{ {z:\,{\mathop{\rm Re} olimits} (1 - i)z) = \sqrt 2 \,} \right\}A={z:Imolimitsz≥1}B={z:∣z−2−i∣=3}C={z:Reolimits(1−i)z)=2​}Let z be any point in A∩B∩CA \cap B \cap CA∩B∩C Then, ∣z+1−i∣2+∣z−5−i∣2{\left| {z + 1 - i} \right|^2} + {\left| {z - 5 - i} \right|^2}∣z+1−i∣2+∣z−5−i∣2 lies between :
  1. A
    25 and 29
  2. B
    30 and 34
  3. C
    35 and 39
  4. D
    40 and 44
View written solutionFree

Correct answer: C

Step-by-step Solution:

1. Understand the definitions of the sets A, B, and C.

Let a complex number be represented as z = x + iy, where x and y are real numbers.

  • Set A: A={z: Im z ≥ 1}A = \left\{ {z:\,{\mathop{\rm Im}\nolimits} \,z\, \ge \,1} \right\}A={z:Imz≥1} The imaginary part of z is y. So, this condition is y≥1y \ge 1y≥1. Geometrically, this represents the upper half-plane including the line y = 1.

  • Set B: B={z:  ∣z−2−i∣=3}B = \left\{ {z:\,\,\left| {z - 2 - i} \right| = 3} \right\}B={z:∣z−2−i∣=3} This can be written as |z - (2 + i)| = 3. This represents all points z in the complex plane whose distance from the point (2 + i) is 3. Geometrically, this is a circle with center CB=(2,1)C_B = (2, 1)CB​=(2,1) and radius r = 3. The equation in Cartesian coordinates is (x−2)2+(y−1)2=32=9(x - 2)^2 + (y - 1)^2 = 3^2 = 9(x−2)2+(y−1)2=32=9.

  • Set C: C={z: Re(1−i)z)=2 }C = \left\{ {z:\,{\mathop{\rm Re}\nolimits} (1 - i)z) = \sqrt 2 \,} \right\}C={z:Re(1−i)z)=2​} Let's compute (1 - i)z: (1−i)(x+iy)=x+iy−ix−i2y=x+y+i(y−x)(1 - i)(x + iy) = x + iy - ix - i^2y = x + y + i(y - x)(1−i)(x+iy)=x+iy−ix−i2y=x+y+i(y−x) The real part is Re((1 - i)z) = x + y. So, the condition is x+y=2x + y = \sqrt 2x+y=2​. Geometrically, this is a straight line.

2. Analyze the expression to be evaluated.

The expression is E=∣z+1−i∣2+∣z−5−i∣2E = |z + 1 - i|^2 + |z - 5 - i|^2E=∣z+1−i∣2+∣z−5−i∣2. Let's express this in terms of x and y: $z + 1 - i = (x + 1) + i(y - 1)$ ∣z+1−i∣2=(x+1)2+(y−1)2|z + 1 - i|^2 = (x + 1)^2 + (y - 1)^2∣z+1−i∣2=(x+1)2+(y−1)2

$z - 5 - i = (x - 5) + i(y - 1)$ ∣z−5−i∣2=(x−5)2+(y−1)2|z - 5 - i|^2 = (x - 5)^2 + (y - 1)^2∣z−5−i∣2=(x−5)2+(y−1)2

Now, sum these two parts: E=[(x+1)2+(y−1)2]+[(x−5)2+(y−1)2]E = [(x + 1)^2 + (y - 1)^2] + [(x - 5)^2 + (y - 1)^2]E=[(x+1)2+(y−1)2]+[(x−5)2+(y−1)2] E=(x2+2x+1)+(y−1)2+(x2−10x+25)+(y−1)2E = (x^2 + 2x + 1) + (y - 1)^2 + (x^2 - 10x + 25) + (y - 1)^2E=(x2+2x+1)+(y−1)2+(x2−10x+25)+(y−1)2 E=2x2−8x+26+2(y−1)2E = 2x^2 - 8x + 26 + 2(y - 1)^2E=2x2−8x+26+2(y−1)2

Let's complete the square for the terms involving x: E=2(x2−4x)+26+2(y−1)2E = 2(x^2 - 4x) + 26 + 2(y - 1)^2E=2(x2−4x)+26+2(y−1)2 E=2(x2−4x+4−4)+26+2(y−1)2E = 2(x^2 - 4x + 4 - 4) + 26 + 2(y - 1)^2E=2(x2−4x+4−4)+26+2(y−1)2 E=2((x−2)2−4)+26+2(y−1)2E = 2((x - 2)^2 - 4) + 26 + 2(y - 1)^2E=2((x−2)2−4)+26+2(y−1)2 E=2(x−2)2−8+26+2(y−1)2E = 2(x - 2)^2 - 8 + 26 + 2(y - 1)^2E=2(x−2)2−8+26+2(y−1)2 E=2(x−2)2+2(y−1)2+18E = 2(x - 2)^2 + 2(y - 1)^2 + 18E=2(x−2)2+2(y−1)2+18 E=2[(x−2)2+(y−1)2]+18E = 2 \left[ (x - 2)^2 + (y - 1)^2 \right] + 18E=2[(x−2)2+(y−1)2]+18

3. Use the condition from Set B.

Any point z in the intersection A∩B∩CA \cap B \cap CA∩B∩C must belong to set B. The condition for z to be in B is (x−2)2+(y−1)2=9(x - 2)^2 + (y - 1)^2 = 9(x−2)2+(y−1)2=9.

Substituting this into our simplified expression for E: $E = 2(9) + 18 = 18 + 18 = 36$

This means that for any point z on the circle defined by set B, the value of the expression is exactly 36.

4. Verify that the intersection A∩B∩CA \cap B \cap CA∩B∩C is not empty.

We need to ensure there exists at least one point z that satisfies the conditions for all three sets. We need to find if the line x+y=2x + y = \sqrt 2x+y=2​ (from C) intersects the circle (x−2)2+(y−1)2=9(x-2)^2 + (y-1)^2 = 9(x−2)2+(y−1)2=9 (from B) in the region y≥1y \ge 1y≥1 (from A).

  • Intersection of B and C: The distance d from the center of the circle (2, 1) to the line x+y−2=0x + y - \sqrt 2 = 0x+y−2​=0 is: d=∣2+1−2∣12+12=∣3−2∣2=3−22d = \frac{|2 + 1 - \sqrt 2|}{\sqrt{1^2 + 1^2}} = \frac{|3 - \sqrt 2|}{\sqrt 2} = \frac{3 - \sqrt 2}{\sqrt 2}d=12+12​∣2+1−2​∣​=2​∣3−2​∣​=2​3−2​​ Since 2≈1.414\sqrt 2 \approx 1.4142​≈1.414, d≈(3−1.414)/1.414≈1.586/1.414≈1.12d \approx (3 - 1.414) / 1.414 \approx 1.586 / 1.414 \approx 1.12d≈(3−1.414)/1.414≈1.586/1.414≈1.12. The radius of the circle is r = 3. Since d < r, the line intersects the circle at two distinct points.

  • Checking condition A: We need to see if at least one of these intersection points has a y-coordinate greater than or equal to 1. The center of the circle is at y=1. The line x+y=2x+y=\sqrt 2x+y=2​ passes through the point (2−1,1)(\sqrt 2 - 1, 1)(2​−1,1). Since 2−1≈0.414<2\sqrt 2 - 1 \approx 0.414 < 22​−1≈0.414<2, and the line has a slope of -1, one part of the line segment chord inside the circle will be above y=1 and the other below. Thus, one of the intersection points will have y > 1. Therefore, the intersection A∩B∩CA \cap B \cap CA∩B∩C is non-empty.

5. Final Conclusion.

Since a point z exists in A∩B∩CA \cap B \cap CA∩B∩C, and for any such point (as it must be in B), the expression evaluates to 36. We need to find which of the given intervals contains the value 36.

  • A: 25 and 29 (25 < 36 < 29 is false)
  • B: 30 and 34 (30 < 36 < 34 is false)
  • C: 35 and 39 (35 < 36 < 39 is true)
  • D: 40 and 44 (40 < 36 < 44 is false)

The value 36 lies between 35 and 39.

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