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Sum of Squared Moduli of Roots of Complex Quadratic Equation

Let z1,z2Cz_1, z_2 \in \mathbb{C} be the distinct solutions of the equation z2+4z(1+12i)=0z^2 + 4z - (1 + 12i) = 0. Then z12+z22|z_1|^2 + |z_2|^2 is equal to :

Options

A

18

B

22

C

29

D

34

Correct

Step-by-Step Solution

To find the sum of the squared moduli of the roots of the quadratic equation z2+4z(1+12i)=0z^2 + 4z - (1 + 12i) = 0, we can complete the square for zz:

(z+2)24112i=0(z + 2)^2 - 4 - 1 - 12i = 0 (z+2)2=5+12i(z + 2)^2 = 5 + 12i

Let w=z+2=x+iyw = z + 2 = x + iy, where x,yRx, y \in \mathbb{R}. Then: (x+iy)2=5+12i(x + iy)^2 = 5 + 12i (x2y2)+2ixy=5+12i(x^2 - y^2) + 2ixy = 5 + 12i

Equating the real and imaginary parts:

  1. x2y2=5x^2 - y^2 = 5
  2. 2xy=12    xy=62xy = 12 \implies xy = 6

Using the identity (x2+y2)2=(x2y2)2+(2xy)2(x^2 + y^2)^2 = (x^2 - y^2)^2 + (2xy)^2, we get: (x2+y2)2=52+122=25+144=169(x^2 + y^2)^2 = 5^2 + 12^2 = 25 + 144 = 169 x2+y2=13x^2 + y^2 = 13

Adding x2y2=5x^2 - y^2 = 5 and x2+y2=13x^2 + y^2 = 13: 2x2=18    x2=9    x=±32x^2 = 18 \implies x^2 = 9 \implies x = \pm 3

Subtracting x2y2=5x^2 - y^2 = 5 from x2+y2=13x^2 + y^2 = 13: 2y2=8    y2=4    y=±22y^2 = 8 \implies y^2 = 4 \implies y = \pm 2

Since xy=6>0xy = 6 > 0, xx and yy must have the same sign. Therefore, w=z+2=±(3+2i)w = z + 2 = \pm (3 + 2i).

This gives us the two roots z1z_1 and z2z_2: z1=(3+2i)2=1+2iz_1 = (3 + 2i) - 2 = 1 + 2i z2=(32i)2=52iz_2 = (-3 - 2i) - 2 = -5 - 2i

Now, calculating the squared moduli of the roots: z12=12+22=1+4=5|z_1|^2 = 1^2 + 2^2 = 1 + 4 = 5 z22=(5)2+(2)2=25+4=29|z_2|^2 = (-5)^2 + (-2)^2 = 25 + 4 = 29

Sum of squared moduli: z12+z22=5+29=34|z_1|^2 + |z_2|^2 = 5 + 29 = 34

Thus, the correct option is D.

Sum of Squared Moduli of Roots of Complex Quadratic Equation | Mathematics PYQ Solution - JEE Challenger