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Complex Numbers question

2017 · Shift 0 · Q30
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Complex Numbers question

2017 · Shift 0 · Q30

JEE MainMathematicsComplex NumbersMCQ+4 / −1
Let ω\omegaω be a complex number such that 2 ω\omegaω+ 1 = z where z =−3\sqrt {-3}−3​. If ∣1111−ω2−1ω21ω2ω7∣=3k\left| {\begin{matrix} 1 & 1 & 1 \\ 1 & { - {\omega ^2} - 1} & {{\omega ^2}} \\ 1 & {{\omega ^2}} & {{\omega ^7}} \\ \end{matrix} } \right| = 3k​111​1−ω2−1ω2​1ω2ω7​​=3k, then k is equal to :
  1. A
    z
  2. B
    -1
  3. C
    1
  4. D
    -z
View written solutionFree

Correct answer: D

  1. Interpret the given condition

    We are given 2ω+1=z,z=−3=i3.2\omega + 1 = z, \quad z = \sqrt{-3} = i\sqrt{3}.2ω+1=z,z=−3​=i3​.

    Hence 2ω=z−1  ⟹  ω=z−12.2\omega = z-1 \implies \omega = \frac{z-1}{2}.2ω=z−1⟹ω=2z−1​.

    Since z=i3z=i\sqrt{3}z=i3​, this is ω=−1+i32.\omega = \frac{-1+i\sqrt{3}}{2}.ω=2−1+i3​​.

    This is a cube root of unity: ω3=1,ω≠1.\omega^3=1, \quad \omega\ne 1.ω3=1,ω=1. Therefore, 1+ω+ω2=0,ω2+ω=−1.1+\omega+\omega^2=0, \quad \omega^2+\omega=-1.1+ω+ω2=0,ω2+ω=−1.

  2. Reduce higher powers

    Because ω3=1\omega^3=1ω3=1, ω7=ω6+1=ω.\omega^7=\omega^{6+1}=\omega.ω7=ω6+1=ω.

    So the determinant becomes

    1 & 1 & 1 \\ 1 & -\omega^2-1 & \omega^2 \\ 1 & \omega^2 & \omega \end{vmatrix}.$$
  3. Use the relation −ω2−1=ω-\omega^2-1=\omega−ω2−1=ω

    From 1+ω+ω2=0  ⟹  −ω2−1=ω.1+\omega+\omega^2=0 \implies -\omega^2-1=\omega.1+ω+ω2=0⟹−ω2−1=ω.

    Thus

    1 & 1 & 1 \\ 1 & \omega & \omega^2 \\ 1 & \omega^2 & \omega \end{vmatrix}.$$
  4. Evaluate the determinant

    Apply row operations: R2→R2−R1,R3→R3−R1.R_2 \to R_2-R_1, \quad R_3 \to R_3-R_1.R2​→R2​−R1​,R3​→R3​−R1​.

    Then

    1 & 1 & 1 \\ 0 & \omega-1 & \omega^2-1 \\ 0 & \omega^2-1 & \omega-1 \end{vmatrix}.$$ Expanding along the first column, $$D=\begin{vmatrix} \omega-1 & \omega^2-1 \\ \omega^2-1 & \omega-1 \end{vmatrix}.$$ So $$D=(\omega-1)^2-(\omega^2-1)^2.$$ Factor as difference of squares: $$D=\big[(\omega-1)-(\omega^2-1)\big]\big[(\omega-1)+(\omega^2-1)\big].$$ That is, $$D=(\omega-\omega^2)(\omega+\omega^2-2).$$ Using $$\omega+\omega^2=-1,$$ we get $$D=(\omega-\omega^2)(-3).$$ Hence $$D=-3(\omega-\omega^2)=3(\omega^2-\omega).$$
  5. Find ω2−ω\omega^2-\omegaω2−ω

    For cube roots of unity, ω=−1+i32,ω2=−1−i32.\omega=\frac{-1+i\sqrt{3}}{2}, \quad \omega^2=\frac{-1-i\sqrt{3}}{2}.ω=2−1+i3​​,ω2=2−1−i3​​.

    Therefore, ω2−ω=−1−i32−−1+i32=−i3.\omega^2-\omega=\frac{-1-i\sqrt{3}}{2}-\frac{-1+i\sqrt{3}}{2}=-i\sqrt{3}.ω2−ω=2−1−i3​​−2−1+i3​​=−i3​.

    Since z=i3,z=i\sqrt{3},z=i3​, we have ω2−ω=−z.\omega^2-\omega=-z.ω2−ω=−z.

    Therefore, D=3(−z).D=3(-z).D=3(−z).

    Comparing with D=3k,D=3k,D=3k, we get k=−z.k=-z.k=−z.

  6. Match with the options

    k=−zk=-zk=−z corresponds to Option D.


Comparison with stored answer: Stored correct answer is D, which matches our result.

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