JEE Challenger
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Area of Rectangle Formed by Roots of Complex Equation

Let z=x+iyz = x + iy be a complex number where xx and yy are integers. Then the area of the rectangle whose vertices are the roots of the equation
zzˉ3+zˉz3=350z \bar{z}^3 + \bar{z} z^3 = 350
is

Options

A

48

Correct
B

32

C

40

D

80

Step-by-Step Solution

To find the area of the rectangle formed by the roots of the equation, we begin by simplifying the given equation:
zzˉ3+zˉz3=350z \bar{z}^3 + \bar{z} z^3 = 350

Factoring out zzˉz\bar{z}, we get:
zzˉ(zˉ2+z2)=350z\bar{z}\left(\bar{z}^2 + z^2\right) = 350

Given z=x+iyz = x + iy where x,y∈Zx, y \in \mathbb{Z}, we have:
zzˉ=∣z∣2=x2+y2z\bar{z} = |z|^2 = x^2 + y^2
z2+zˉ2=(x+iy)2+(x−iy)2=2(x2−y2)z^2 + \bar{z}^2 = (x + iy)^2 + (x - iy)^2 = 2(x^2 - y^2)

Substituting these expressions back into the equation:
(x2+y2)⋅2(x2−y2)=350(x^2 + y^2) \cdot 2(x^2 - y^2) = 350
2(x4−y4)=3502(x^4 - y^4) = 350
x4−y4=175x^4 - y^4 = 175

Factoring the difference of squares:
(x2−y2)(x2+y2)=175(x^2 - y^2)(x^2 + y^2) = 175

Since xx and yy are integers, x2x^2 and y2y^2 are non-negative integers. Therefore, x2+y2>0x^2 + y^2 > 0 and x2+y2≥x2−y2x^2 + y^2 \ge x^2 - y^2.
The positive integer factor pairs of 175175 are (1,175)(1, 175), (5,35)(5, 35), and (7,25)(7, 25).

We test each pair:

  1. If x2−y2=1x^2 - y^2 = 1 and x2+y2=175x^2 + y^2 = 175:
    2x2=176  ⟹  x2=88(not a perfect square)2x^2 = 176 \implies x^2 = 88 \quad (\text{not a perfect square})

  2. If x2−y2=5x^2 - y^2 = 5 and x2+y2=35x^2 + y^2 = 35:
    2x2=40  ⟹  x2=20(not a perfect square)2x^2 = 40 \implies x^2 = 20 \quad (\text{not a perfect square})

  3. If x2−y2=7x^2 - y^2 = 7 and x2+y2=25x^2 + y^2 = 25:
    2x2=32  ⟹  x2=16  ⟹  x=±42x^2 = 32 \implies x^2 = 16 \implies x = \pm 4
    2y2=18  ⟹  y2=9  ⟹  y=±32y^2 = 18 \implies y^2 = 9 \implies y = \pm 3
    Both xx and yy are integers.

Thus, the four vertices of the rectangle in the Argand plane are:
(4,3),(4,−3),(−4,3),(−4,−3)(4, 3), \quad (4, -3), \quad (-4, 3), \quad (-4, -3)

The dimensions of the rectangle are:

  • Length along the real axis: 4−(−4)=84 - (-4) = 8
  • Breadth along the imaginary axis: 3−(−3)=63 - (-3) = 6

Therefore, the area of the rectangle is:
Area=8×6=48\text{Area} = 8 \times 6 = 48

Hence, the correct option is A.