JEE MainMathematicsComplex NumbersMCQ+4 / −1
Let be a complex number such that 2 + 1 = z where z =. If , then k is equal to :
- Az
- B-1
- C1
- D-z
View written solutionFree
Correct answer: D
-
Interpret the given condition
We are given
Hence
Since , this is
This is a cube root of unity: Therefore,
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Reduce higher powers
Because ,
So the determinant becomes
1 & 1 & 1 \\ 1 & -\omega^2-1 & \omega^2 \\ 1 & \omega^2 & \omega \end{vmatrix}.$$ -
Use the relation
From
Thus
1 & 1 & 1 \\ 1 & \omega & \omega^2 \\ 1 & \omega^2 & \omega \end{vmatrix}.$$ -
Evaluate the determinant
Apply row operations:
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).$$ -
Find
For cube roots of unity,
Therefore,
Since we have
Therefore,
Comparing with we get
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Match with the options
corresponds to Option D.
Comparison with stored answer: Stored correct answer is D, which matches our result.
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