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JEE Advanced 2009 · Complex Numbers and Quadratic Equations

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

Answer: A. 4848

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Written solution

z=x+iy,x,y∈Zz = x + iy, \quad x, y \in \mathbb{Z}

zzˉ3+zˉz3=350z\bar{z}^{3} + \bar{z}z^{3} = 350

To find: the area of the rectangle whose vertices are the roots\text{To find: the area of the rectangle whose vertices are the roots}

z=reiθ,zˉ=re−iθz = re^{i\theta}, \quad \bar{z} = re^{-i\theta}

zˉ3=r3e−3iθ,z3=r3e3iθ\bar{z}^{3} = r^{3}e^{-3i\theta}, \quad z^{3} = r^{3}e^{3i\theta}

zzˉ3=r4e−2iθ,zˉz3=r4e2iθz\bar{z}^{3} = r^{4}e^{-2i\theta}, \quad \bar{z}z^{3} = r^{4}e^{2i\theta}

zzˉ3+zˉz3=r4(e2iθ+e−2iθ)=2r4cos⁡2θz\bar{z}^{3} + \bar{z}z^{3} = r^{4}\left(e^{2i\theta} + e^{-2i\theta}\right) = 2r^{4}\cos 2\theta

2r4cos⁡2θ=350 ⇒ r4cos⁡2θ=1752r^{4}\cos 2\theta = 350 \ \Rightarrow\ r^{4}\cos 2\theta = 175

r2=x2+y2r^{2} = x^{2} + y^{2}

r2cos⁡2θ=x2−y2r^{2}\cos 2\theta = x^{2} - y^{2}

r4cos⁡2θ=(x2+y2)(x2−y2)=175r^{4}\cos 2\theta = (x^{2} + y^{2})(x^{2} - y^{2}) = 175

x4−y4=175x^{4} - y^{4} = 175

(x2−y2)(x2+y2)=175(x^{2} - y^{2})(x^{2} + y^{2}) = 175

a=x2−y2,b=x2+y2a = x^{2} - y^{2}, \quad b = x^{2} + y^{2}

ab=175,b>a>0ab = 175, \quad b > a > 0

x2=a+b2,y2=b−a2x^{2} = \frac{a + b}{2}, \quad y^{2} = \frac{b - a}{2}

175=1×175=5×35=7×25175 = 1 \times 175 = 5 \times 35 = 7 \times 25

(1,175): x2=1762=88 ✗(1,175):\ x^{2} = \frac{176}{2} = 88\ \text{✗}

(5,35): x2=402=20 ✗(5,35):\ x^{2} = \frac{40}{2} = 20\ \text{✗}

(7,25): x2=322=16 ✓(7,25):\ x^{2} = \frac{32}{2} = 16\ \checkmark

x2=16 ⇒ x=±4x^{2} = 16 \ \Rightarrow\ x = \pm 4

y2=25−72=9 ⇒ y=±3y^{2} = \frac{25 - 7}{2} = 9 \ \Rightarrow\ y = \pm 3

z=±4±3iz = \pm 4 \pm 3i

width=4−(−4)=8,height=3−(−3)=6\text{width} = 4 - (-4) = 8, \quad \text{height} = 3 - (-3) = 6

Area=8×6=48\text{Area} = 8 \times 6 = 48

Area=48⇒option (1)\boxed{\text{Area} = 48} \quad \Rightarrow \text{option (1)}

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