arthham

JEE Advanced 2010 · Determinants

Let ω\omega be the complex number cos⁡2π3+isin⁡2π3\cos \frac{2\pi}{3}+i\sin \frac{2\pi}{3}. Then the number of distinct complex numbers zz satisfying ∣z+1ωω2ωz+ω21ω21z+ω∣=0\begin{vmatrix} z+1 & \omega & \omega^{2} \\ \omega & z+\omega^{2} & 1 \\ \omega^{2} & 1 & z+\omega \end{vmatrix}=0 is equal to

Answer: 00

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Written solution

ω=e2πi/3,ω3=1\omega = e^{2\pi i/3},\quad \omega^{3}=1

1+ω+ω2=01+\omega+\omega^{2}=0

Find the number of distinct z with\text{Find the number of distinct } z \text{ with}

z+1 ω ω² ω z+ω² 1 ω² 1 z+ω = 0

1. Write M=zI+A\text{1. Write } M = zI + A

2. Study A using 1+ω+ω2=0, ω3=1\text{2. Study } A \text{ using } 1+\omega+\omega^{2}=0,\ \omega^{3}=1

3. Reduce det⁡(zI+A) and count distinct z\text{3. Reduce } \det(zI+A) \text{ and count distinct } z

M = zI + A, A = 1 ω ω² ω ω² 1 ω² 1 ω

Row sums =1+ω+ω2=0⇒det⁡A=0\text{Row sums } = 1+\omega+\omega^{2}=0 \Rightarrow \det A = 0

tr⁡A=1+ω2+ω=0\operatorname{tr}A = 1+\omega^{2}+\omega = 0

A2=0A^{2} = 0

det⁡(zI+A)=z3+(tr⁡A)z2+c2z+det⁡A\det(zI+A) = z^{3} + (\operatorname{tr}A)z^{2} + c_{2}z + \det A

tr⁡A=det⁡A=0,c2=0⇒det⁡(zI+A)=z3\operatorname{tr}A = \det A = 0,\quad c_{2} = 0 \Rightarrow \det(zI+A) = z^{3}

z3=0⇒z=0 (multiplicity 3)z^{3}=0 \Rightarrow z=0\ \text{(multiplicity } 3\text{)}

Answer: 1\boxed{\text{Answer: } 1}

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