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Complex Numbers question

2009 · Shift 1 · Q21
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  5. /2009 · Shift 1 · Q21

Complex Numbers question

2009 · Shift 1 · Q21

JEE AdvancedMathematicsComplex NumbersMCQ+3 / −1
Let z=x+iyz = x + iyz=x+iy be a complex number where x and y are integers. Then the area of the rectangle whose vertices are the roots of the equation z‾z3+zz‾3=350\overline z {z^3} + z{\overline z ^3} = 350zz3+zz3=350 is
  1. A
    48
  2. B
    32
  3. C
    40
  4. D
    80
View written solutionFree

Correct answer: A

Step-by-step Solution:

  1. Analyze the given equation: We are given the equation z‾z3+zz‾3=350\overline z {z^3} + z{\overline z ^3} = 350zz3+zz3=350, where z=x+iyz = x + iyz=x+iy and x,yx, yx,y are integers.

  2. Simplify the equation: We can factor out the common term zz‾z\overline zzz from the left-hand side of the equation. zz‾(z2+z‾2)=350z\overline z (z^2 + {\overline z}^2) = 350zz(z2+z2)=350 We know that zz‾=∣z∣2z\overline z = |z|^2zz=∣z∣2. For z=x+iyz = x+iyz=x+iy, this is x2+y2x^2+y^2x2+y2. So the equation becomes: ∣z∣2(z2+z‾2)=350|z|^2 (z^2 + {\overline z}^2) = 350∣z∣2(z2+z2)=350

  3. Express the terms in x and y:

    • ∣z∣2=x2+y2|z|^2 = x^2+y^2∣z∣2=x2+y2
    • To find z2+z‾2z^2 + {\overline z}^2z2+z2, we can use the identity z2+z‾2=(z+z‾)2−2zz‾z^2 + {\overline z}^2 = (z+\overline z)^2 - 2z\overline zz2+z2=(z+z)2−2zz. We know z+z‾=(x+iy)+(x−iy)=2xz+\overline z = (x+iy) + (x-iy) = 2xz+z=(x+iy)+(x−iy)=2x. So, z2+z‾2=(2x)2−2∣z∣2=4x2−2(x2+y2)=2x2−2y2=2(x2−y2)z^2 + {\overline z}^2 = (2x)^2 - 2|z|^2 = 4x^2 - 2(x^2+y^2) = 2x^2 - 2y^2 = 2(x^2 - y^2)z2+z2=(2x)2−2∣z∣2=4x2−2(x2+y2)=2x2−2y2=2(x2−y2).
  4. Substitute back into the simplified equation: Substituting the expressions for ∣z∣2|z|^2∣z∣2 and z2+z‾2z^2 + {\overline z}^2z2+z2 into our simplified equation: (x2+y2)[2(x2−y2)]=350(x^2+y^2) [2(x^2-y^2)] = 350(x2+y2)[2(x2−y2)]=350 (x2+y2)(x2−y2)=175(x^2+y^2)(x^2-y^2) = 175(x2+y2)(x2−y2)=175

  5. Solve for the integer values of x and y: Let A=x2A = x^2A=x2 and B=y2B = y^2B=y2. Since xxx and yyy are integers, AAA and BBB must be non-negative integers (specifically, perfect squares). The equation becomes: (A+B)(A−B)=175(A+B)(A-B) = 175(A+B)(A−B)=175 We need to find pairs of factors of 175. The factors of 175 are 1, 5, 7, 25, 35, 175. Let A+B=uA+B = uA+B=u and A−B=vA-B = vA−B=v, where uv=175uv=175uv=175. Since y2=B≥0y^2 = B \ge 0y2=B≥0, we have A+B≥A−BA+B \ge A-BA+B≥A−B, which means u≥vu \ge vu≥v. Also, for A=(u+v)/2A = (u+v)/2A=(u+v)/2 and B=(u−v)/2B=(u-v)/2B=(u−v)/2 to be integers, uuu and vvv must have the same parity. Since 175 is odd, both uuu and vvv must be odd, which is true for all factors of 175.

    Let's check the possible pairs (u,v)(u, v)(u,v):

    • Case 1: u=175,v=1u=175, v=1u=175,v=1 A+B=175A+B = 175A+B=175 and A−B=1A-B = 1A−B=1. Adding them: 2A=176  ⟹  A=882A = 176 \implies A = 882A=176⟹A=88. So x2=88x^2 = 88x2=88. This is not possible for an integer xxx.

    • Case 2: u=35,v=5u=35, v=5u=35,v=5 A+B=35A+B = 35A+B=35 and A−B=5A-B = 5A−B=5. Adding them: 2A=40  ⟹  A=202A = 40 \implies A = 202A=40⟹A=20. So x2=20x^2 = 20x2=20. This is not possible for an integer xxx.

    • Case 3: u=25,v=7u=25, v=7u=25,v=7 A+B=25A+B = 25A+B=25 and A−B=7A-B = 7A−B=7. Adding them: 2A=32  ⟹  A=162A = 32 \implies A = 162A=32⟹A=16. So x2=16  ⟹  x=±4x^2 = 16 \implies x = \pm 4x2=16⟹x=±4. Subtracting them: 2B=18  ⟹  B=92B = 18 \implies B = 92B=18⟹B=9. So y2=9  ⟹  y=±3y^2 = 9 \implies y = \pm 3y2=9⟹y=±3. This case gives integer solutions for xxx and yyy.

  6. Identify the roots of the equation: The integer solutions for (x,y)(x, y)(x,y) are (±4,±3)(\pm 4, \pm 3)(±4,±3). This gives us four complex numbers z=x+iyz=x+iyz=x+iy which are the roots of the equation:

    • z1=4+3iz_1 = 4 + 3iz1​=4+3i
    • z2=4−3iz_2 = 4 - 3iz2​=4−3i
    • z3=−4+3iz_3 = -4 + 3iz3​=−4+3i
    • z4=−4−3iz_4 = -4 - 3iz4​=−4−3i
  7. Calculate the area of the rectangle: The vertices of the rectangle in the Argand plane are the points corresponding to these roots: (4,3),(4,−3),(−4,3),(4, 3), (4, -3), (-4, 3),(4,3),(4,−3),(−4,3), and (−4,−3)(-4, -3)(−4,−3). The length of the rectangle is the distance between the points (4,3)(4,3)(4,3) and (−4,3)(-4,3)(−4,3), which is ∣4−(−4)∣=8|4 - (-4)| = 8∣4−(−4)∣=8. The width of the rectangle is the distance between the points (4,3)(4,3)(4,3) and (4,−3)(4,-3)(4,−3), which is ∣3−(−3)∣=6|3 - (-3)| = 6∣3−(−3)∣=6. The area of the rectangle is length ×\times× width. Area=8×6=48\text{Area} = 8 \times 6 = 48Area=8×6=48

  8. Conclusion: The area of the rectangle is 48. This corresponds to option A.

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