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JEE Main 20238 AprilMorning Shift
Exam
JEE Main
Year
2023
Date
8 April
Shift
Morning Shift
Subject
Mathematics
Chapter
Complex Number

JEE Main 2023 · 8 April · Morning Shift · Mathematics · Complex Number

If for z = α + iβ, |z + 2| = z + 4 ( 1 + i ) , then α + β and αβ...

If for z=α+iβ,∣z+2∣=z+4(1+i),z = \alpha + i\beta, \left|z + 2\right| = z + 4 ( 1 + i ) , then α+β\alpha + \beta and αβ\alpha\beta are the roots of the equation

Options

  1. Ax2+3x−4=0{x}^{2} + 3x - 4 = 0
  2. Bx2+7x+12=0{x}^{2} + 7x + 12 = 0
  3. Cx2+x−12=0{x}^{2} + x - 12 = 0
  4. Dx2+2x−3=0{x}^{2} + 2x - 3 = 0

Correct answer

B. x2+7x+12=0{x}^{2} + 7x + 12 = 0

Step-by-step solution

Given,

z=α+iβ and ∣z+2∣=z+4(1+i)z = \alpha + i\beta \text{ and } \left|z + 2\right| = z + 4\left(1 + i\right)

Now putting the value of z=α+iβz = \alpha + i\beta in \left|z + 2\right| = z + 4\left(1 + i\right)weget,</p><p>∣z+2∣=z+4(1+i)we get,</p><p>\left|z + 2\right| = z + 4\left(1 + i\right)

⇒(α+2)2+β2=(α+4)+i(β+4)⇒ \sqrt{{\left(\alpha + 2\right)}^{2} + {\beta}^{2}} = \left(\alpha + 4\right) + i\left(\beta + 4\right)

Now on comparing real and imaginary part, we get

(α+2)2+β2=α+4...(i)\sqrt{( \alpha + 2{)}^{2} + {\beta}^{2}} = \alpha + 4 . . . ( i ) and β+4=0⇒β=−4...(ii)\beta + 4 = 0 ⇒ \beta = - 4 . . . ( ii )

Now solving,

(α+2)2+16=α+4\sqrt{( \alpha + 2{)}^{2} + 16} = \alpha + 4

⇒α2+4α+20=α2+8α+16⇒ {\alpha}^{2} + 4\alpha + 20 = {\alpha}^{2} + 8\alpha + 16

⇒α=1⇒ \alpha = 1

So, α+β=−3,αβ=−4\alpha + \beta = - 3, \alpha\beta = - 4

We know that quadratic equation is given by,

x2−(sum of roots)x+(product of roots)=0{x}^{2} - \left(\text{sum of roots}\right)x + \left(\text{product of roots}\right) = 0

So, equation with roots −3- 3 and −4- 4 will be,

x2+7x+12=0{x}^{2} + 7x + 12 = 0

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