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Complex Numbers question

2011 · Shift 2 · Q22
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Complex Numbers question

2011 · Shift 2 · Q22

JEE AdvancedMathematicsComplex NumbersNumerical+4 / −1
Let ω=eiπ3\omega = {e^{{{i\pi } \over 3}}}ω=e3iπ​, and a, b, c, x, y, z be non-zero complex numbers such that a+b+c=xa+bω+cω2=ya+bω2+cω=za + b + c = xa + b\omega + c{\omega ^2} = ya + b{\omega ^2} + c\omega = za+b+c=xa+bω+cω2=ya+bω2+cω=z Then the value of ∣x∣2+∣y∣2+∣z∣2∣a∣2+∣b∣2+∣c∣2{{{{\left| x \right|}^2} + {{\left| y \right|}^2} + {{\left| z \right|}^2}} \over {{{\left| a \right|}^2} + {{\left| b \right|}^2} + {{\left| c \right|}^2}}}∣a∣2+∣b∣2+∣c∣2∣x∣2+∣y∣2+∣z∣2​ is
Numerical answer
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Correct answer: 3

  1. We are given
ω=eiπ/3,a+b+c=z,\omega=e^{i\pi/3},\qquad a+b+c=z,ω=eiπ/3,a+b+c=z,

and also

xa+bω+cω2=z,xa+b\omega+c\omega^2=z,xa+bω+cω2=z, ya+bω2+cω=z.ya+b\omega^2+c\omega=z.ya+bω2+cω=z.

So all three expressions are equal:

a+b+c=xa+bω+cω2=ya+bω2+cω=z.a+b+c=xa+b\omega+c\omega^2=ya+b\omega^2+c\omega=z.a+b+c=xa+bω+cω2=ya+bω2+cω=z.

We need to find

∣x∣2+∣y∣2+∣z∣2∣a∣2+∣b∣2+∣c∣2.\frac{|x|^2+|y|^2+|z|^2}{|a|^2+|b|^2+|c|^2}.∣a∣2+∣b∣2+∣c∣2∣x∣2+∣y∣2+∣z∣2​.
  1. Use the first equality:
a+b+c=xa+bω+cω2.a+b+c=xa+b\omega+c\omega^2.a+b+c=xa+bω+cω2.

Rearrange:

a−xa+b−bω+c−cω2=0a-xa+b-b\omega+c-c\omega^2=0a−xa+b−bω+c−cω2=0 a(1−x)+b(1−ω)+c(1−ω2)=0.a(1-x)+b(1-\omega)+c(1-\omega^2)=0.a(1−x)+b(1−ω)+c(1−ω2)=0.

Similarly, from

a+b+c=ya+bω2+cω,a+b+c=ya+b\omega^2+c\omega,a+b+c=ya+bω2+cω,

we get

a(1−y)+b(1−ω2)+c(1−ω)=0.a(1-y)+b(1-\omega^2)+c(1-\omega)=0.a(1−y)+b(1−ω2)+c(1−ω)=0.
  1. Since
ω=eiπ/3=12+i32,\omega=e^{i\pi/3}=\frac12+i\frac{\sqrt3}{2},ω=eiπ/3=21​+i23​​,

we have

1−ω=12−i32=e−iπ/3=ω−1,1-\omega=\frac12-i\frac{\sqrt3}{2}=e^{-i\pi/3}=\omega^{-1},1−ω=21​−i23​​=e−iπ/3=ω−1,

and

1−ω2=1−e2iπ/3=32−i32=3 e−iπ/6.1-\omega^2=1-e^{2i\pi/3}=\frac32-i\frac{\sqrt3}{2}=\sqrt3\,e^{-i\pi/6}.1−ω2=1−e2iπ/3=23​−i23​​=3​e−iπ/6.

But a cleaner way is to solve directly for x,y,zx,y,zx,y,z in terms of a,b,ca,b,ca,b,c.

From

z=a+b+c=xa+bω+cω2,z=a+b+c=xa+b\omega+c\omega^2,z=a+b+c=xa+bω+cω2,

we get

xa=a+b(1−ω)+c(1−ω2),xa=a+b(1-\omega)+c(1-\omega^2),xa=a+b(1−ω)+c(1−ω2),

so

x=1+b(1−ω)+c(1−ω2)a.x=1+\frac{b(1-\omega)+c(1-\omega^2)}{a}.x=1+ab(1−ω)+c(1−ω2)​.

This is not immediately useful for the required ratio.

So instead, let us use a linear transformation viewpoint.


  1. Define the vector
v=(abc).\mathbf{v}=\begin{pmatrix}a\\ b\\ c\end{pmatrix}.v=​abc​​.

Then

z=a+b+c,z=a+b+c,z=a+b+c, xa=z−bω−cω2=a+b(1−ω)+c(1−ω2),xa=z-b\omega-c\omega^2=a+b(1-\omega)+c(1-\omega^2),xa=z−bω−cω2=a+b(1−ω)+c(1−ω2), ya=z−bω2−cω=a+b(1−ω2)+c(1−ω).ya=z-b\omega^2-c\omega=a+b(1-\omega^2)+c(1-\omega).ya=z−bω2−cω=a+b(1−ω2)+c(1−ω).

Hence

(xayaz)=(11−ω1−ω211−ω21−ω111)(abc).\begin{pmatrix}xa\\ ya\\ z\end{pmatrix} = \begin{pmatrix} 1 & 1-\omega & 1-\omega^2\\ 1 & 1-\omega^2 & 1-\omega\\ 1 & 1 & 1 \end{pmatrix} \begin{pmatrix}a\\ b\\ c\end{pmatrix}.​xayaz​​=​111​1−ω1−ω21​1−ω21−ω1​​​abc​​.

But this still contains xa,yaxa,yaxa,ya instead of x,yx,yx,y. So let us find a direct relation by testing whether the quantity is invariant and can be simplified.


  1. Since all variables are non-zero, divide the equations by aaa and set
u=ba,μ=ca.u=\frac{b}{a},\qquad \mu=\frac{c}{a}.u=ab​,μ=ac​.

Then

za=1+ν+μ,\frac{z}{a}=1+\nu+\mu,az​=1+ν+μ, x+νω+μω2=za=1+ν+μ,x+\nu\omega+\mu\omega^2=\frac{z}{a}=1+\nu+\mu,x+νω+μω2=az​=1+ν+μ, y+νω2+μω=1+ν+μ.y+\nu\omega^2+\mu\omega=1+\nu+\mu.y+νω2+μω=1+ν+μ.

Therefore

x=1+ν(1−ω)+μ(1−ω2),x=1+\nu(1-\omega)+\mu(1-\omega^2),x=1+ν(1−ω)+μ(1−ω2), y=1+ν(1−ω2)+μ(1−ω),y=1+\nu(1-\omega^2)+\mu(1-\omega),y=1+ν(1−ω2)+μ(1−ω), za=1+ν+μ.\frac{z}{a}=1+\nu+\mu.az​=1+ν+μ.

So

∣x∣2+∣y∣2+∣za∣2|x|^2+|y|^2+\left|\frac{z}{a}\right|^2∣x∣2+∣y∣2+​az​​2

must be compared with

1+∣ν∣2+∣μ∣2=∣a∣2+∣b∣2+∣c∣2∣a∣2.1+|\nu|^2+|\mu|^2=\frac{|a|^2+|b|^2+|c|^2}{|a|^2}.1+∣ν∣2+∣μ∣2=∣a∣2∣a∣2+∣b∣2+∣c∣2​.

Hence the desired ratio becomes

∣x∣2+∣y∣2+∣z∣2∣a∣2+∣b∣2+∣c∣2=∣x∣2+∣y∣2+∣za∣21+∣ν∣2+∣μ∣2.\frac{|x|^2+|y|^2+|z|^2}{|a|^2+|b|^2+|c|^2} = \frac{|x|^2+|y|^2+\left|\frac{z}{a}\right|^2}{1+|\nu|^2+|\mu|^2}.∣a∣2+∣b∣2+∣c∣2∣x∣2+∣y∣2+∣z∣2​=1+∣ν∣2+∣μ∣2∣x∣2+∣y∣2+​az​​2​.
  1. Now use specific values of ω\omegaω:
1−ω=ωˉ,1-\omega=\bar\omega,1−ω=ωˉ,

because ω=eiπ/3\omega=e^{i\pi/3}ω=eiπ/3 and ωˉ=e−iπ/3\bar\omega=e^{-i\pi/3}ωˉ=e−iπ/3. Also,

1−ω2=−ω2.1-\omega^2=-\omega^2.1−ω2=−ω2.

Indeed,

1−ω2=1−e2iπ/3=e−iπ/3=ωˉ?1-\omega^2=1-e^{2i\pi/3}=e^{-i\pi/3}=\bar\omega? 1−ω2=1−e2iπ/3=e−iπ/3=ωˉ?

Let us compute carefully:

ω=12+i32,ω2=−12+i32.\omega=\frac12+i\frac{\sqrt3}{2},\qquad \omega^2=-\frac12+i\frac{\sqrt3}{2}.ω=21​+i23​​,ω2=−21​+i23​​.

Thus

1−ω=12−i32=ω−1=ωˉ,1-\omega=\frac12-i\frac{\sqrt3}{2}=\omega^{-1}=\bar\omega,1−ω=21​−i23​​=ω−1=ωˉ, 1−ω2=32−i32=3 e−iπ/6.1-\omega^2=\frac32-i\frac{\sqrt3}{2}=\sqrt3\,e^{-i\pi/6}.1−ω2=23​−i23​​=3​e−iπ/6.

So direct simplification is messy.

A better route is to use the identity

1+ω2=ω.1+\omega^2=\omega.1+ω2=ω.

Since ω2−ω+1=0\omega^2-\omega+1=0ω2−ω+1=0 for ω=eiπ/3\omega=e^{i\pi/3}ω=eiπ/3. Similarly,

1+ω=ω2+2?1+\omega=\omega^2+2? 1+ω=ω2+2?

Not as useful.

Let us instead use the matrix method with rows

r1=(1,1−ω,1−ω2),r_1=(1,1-\omega,1-\omega^2),r1​=(1,1−ω,1−ω2), r2=(1,1−ω2,1−ω),r_2=(1,1-\omega^2,1-\omega),r2​=(1,1−ω2,1−ω), r3=(1,1,1).r_3=(1,1,1).r3​=(1,1,1).

Then

(xyz/a)=M(1νμ).\begin{pmatrix}x\\y\\z/a\end{pmatrix}=M\begin{pmatrix}1\\\nu\\\mu\end{pmatrix}.​xyz/a​​=M​1νμ​​.

The required ratio is

∥Mu∥2∥u∥2,\frac{\|M u\|^2}{\|u\|^2},∥u∥2∥Mu∥2​,

where

u=u2, μ=u3, u=(1νμ).u=u_2,\ \mu=u_3,\ u=\begin{pmatrix}1\\\nu\\\mu\end{pmatrix}.u=u2​, μ=u3​, u=​1νμ​​.

If the rows of MMM are orthogonal and each has squared norm 333, then

M∗M=3IM^*M=3IM∗M=3I

and hence

∥Mu∥2=3∥u∥2.\|Mu\|^2=3\|u\|^2.∥Mu∥2=3∥u∥2.

Let us verify this.


  1. Compute row norms.

First row:

∥r1∥2=∣1∣2+∣1−ω∣2+∣1−ω2∣2.\|r_1\|^2=|1|^2+|1-\omega|^2+|1-\omega^2|^2.∥r1​∥2=∣1∣2+∣1−ω∣2+∣1−ω2∣2.

Now

∣1−ω∣=2sin⁡π6=1,|1-\omega|=2\sin\frac{\pi}{6}=1,∣1−ω∣=2sin6π​=1,

so

∣1−ω∣2=1.|1-\omega|^2=1.∣1−ω∣2=1.

Also

∣1−ω2∣=2sin⁡2π6=2sin⁡π3=3,|1-\omega^2|=2\sin\frac{2\pi}{6}=2\sin\frac{\pi}{3}=\sqrt3,∣1−ω2∣=2sin62π​=2sin3π​=3​,

so

∣1−ω2∣2=3.|1-\omega^2|^2=3.∣1−ω2∣2=3.

This gives

∥r1∥2=1+1+3=5.\|r_1\|^2=1+1+3=5.∥r1​∥2=1+1+3=5.

So this matrix is not the right one for a unitary-type simplification.

Thus let us use a different transformation.


  1. Observe the equations:
z=a+b+c,z=a+b+c,z=a+b+c, z=xa+bω+cω2,z=xa+b\omega+c\omega^2,z=xa+bω+cω2, z=ya+bω2+cω.z=ya+b\omega^2+c\omega.z=ya+bω2+cω.

Add the last two equations:

2z=(x+y)a+b(ω+ω2)+c(ω+ω2).2z=(x+y)a+b(\omega+\omega^2)+c(\omega+\omega^2).2z=(x+y)a+b(ω+ω2)+c(ω+ω2).

Since

ω+ω2=i3,\omega+\omega^2=i\sqrt3,ω+ω2=i3​,

this is not directly helpful.

Instead, subtract from the first:

(1−x)a+(1−ω)b+(1−ω2)c=0...(1)(1-x)a+(1-\omega)b+(1-\omega^2)c=0 \quad ...(1)(1−x)a+(1−ω)b+(1−ω2)c=0...(1) (1−y)a+(1−ω2)b+(1−ω)c=0...(2)(1-y)a+(1-\omega^2)b+(1-\omega)c=0 \quad ...(2)(1−y)a+(1−ω2)b+(1−ω)c=0...(2)

Now note that

(1−ω)(1−ω2)=1.(1-\omega)(1-\omega^2)=1.(1−ω)(1−ω2)=1.

Indeed,

(1−ω)(1−ω2)=1−(ω+ω2)+ω3=2−(ω+ω2)=1(1-\omega)(1-\omega^2)=1-(\omega+\omega^2)+\omega^3=2-(\omega+\omega^2)=1(1−ω)(1−ω2)=1−(ω+ω2)+ω3=2−(ω+ω2)=1

using ω3=−1\omega^3=-1ω3=−1 and ω+ω2=i3\omega+\omega^2=i\sqrt3ω+ω2=i3​ is messy; numerically,

1−ω=12−i32,1−ω2=32−i32,1-\omega=\frac12-i\frac{\sqrt3}{2},\quad 1-\omega^2=\frac32-i\frac{\sqrt3}{2},1−ω=21​−i23​​,1−ω2=23​−i23​​,

whose product is not 111. So this is false.

Let us solve for b,cb,cb,c in terms of x,yx,yx,y. Subtract (2) from (1):

(y−x)a+[(1−ω)−(1−ω2)]b+[(1−ω2)−(1−ω)]c=0,(y-x)a+[(1-\omega)-(1-\omega^2)]b+[(1-\omega^2)-(1-\omega)]c=0,(y−x)a+[(1−ω)−(1−ω2)]b+[(1−ω2)−(1−ω)]c=0, (y−x)a+(ω2−ω)b+(ω−ω2)c=0,(y-x)a+(\omega^2-\omega)b+(\omega-\omega^2)c=0,(y−x)a+(ω2−ω)b+(ω−ω2)c=0, (y−x)a+(ω2−ω)(b−c)=0.(y-x)a+(\omega^2-\omega)(b-c)=0.(y−x)a+(ω2−ω)(b−c)=0.

Hence

b−c=(x−y)aω2−ω....(3)b-c=\frac{(x-y)a}{\omega^2-\omega}. \quad ...(3)b−c=ω2−ω(x−y)a​....(3)

Adding (1) and (2):

(2−x−y)a+[2−(ω+ω2)](b+c)=0.(2-x-y)a+[2-(\omega+\omega^2)](b+c)=0.(2−x−y)a+[2−(ω+ω2)](b+c)=0.

Since

ω+ω2=i3,\omega+\omega^2=i\sqrt3,ω+ω2=i3​,

this is again ugly.

This suggests a standard DFT-type identity may be intended with cube roots of unity, but here ω=eiπ/3\omega=e^{i\pi/3}ω=eiπ/3 is a sixth root. Still, let us test by choosing simple values consistent with the equations.


  1. Take a simple case: let b=cb=cb=c. Then from the equations for x,yx,yx,y,
x=1+b/a [(1−ω)+(1−ω2)],x=1+b/a\,[(1-\omega)+(1-\omega^2)],x=1+b/a[(1−ω)+(1−ω2)], y=1+b/a [(1−ω2)+(1−ω)]=x.y=1+b/a\,[(1-\omega^2)+(1-\omega)] = x.y=1+b/a[(1−ω2)+(1−ω)]=x.

Also

za=1+2b/a.\frac{z}{a}=1+2b/a.az​=1+2b/a.

Let t=b/a=c/at=b/a=c/at=b/a=c/a. Then

x=y=1+t(2−ω−ω2),x=y=1+t(2-\omega-\omega^2),x=y=1+t(2−ω−ω2), za=1+2t.\frac{z}{a}=1+2t.az​=1+2t.

Since

ω+ω2=i3,\omega+\omega^2=i\sqrt3,ω+ω2=i3​,

this becomes variable, so ratio would depend on ttt unless some hidden identity exists. But the problem expects a fixed integer, so we should inspect the intended root.

For JEE standard problems of this form, usually ω\omegaω denotes a cube root of unity satisfying

1+ω+ω2=0.1+\omega+\omega^2=0.1+ω+ω2=0.

Then the answer indeed often becomes 333. Given stored answer is 333, the intended interpretation is almost certainly cube root of unity, i.e. ω=e2πi/3\omega=e^{2\pi i/3}ω=e2πi/3, not eiπ/3e^{i\pi/3}eiπ/3.

Let us solve under the standard cube-root identity and verify.


  1. Assume now ω\omegaω is a primitive cube root of unity, so
1+ω+ω2=0,1+\omega+\omega^2=0,1+ω+ω2=0, 1+ω=−ω2,1+\omega=-\omega^2,1+ω=−ω2, 1+ω2=−ω,1+\omega^2=-\omega,1+ω2=−ω, ∣ω∣=1.|\omega|=1.∣ω∣=1.

From

a+b+c=xa+bω+cω2,a+b+c=xa+b\omega+c\omega^2,a+b+c=xa+bω+cω2,

we get

(1−x)a+(1−ω)b+(1−ω2)c=0.(1-x)a+(1-\omega)b+(1-\omega^2)c=0.(1−x)a+(1−ω)b+(1−ω2)c=0.

Using

1−ω=−(ω+2?)1-\omega=-(\omega+2?)1−ω=−(ω+2?)

Instead, directly use DFT inversion.

Let

s0=a+b+c=z,s_0=a+b+c=z,s0​=a+b+c=z, s1=xa+bω+cω2=z,s_1=xa+b\omega+c\omega^2=z,s1​=xa+bω+cω2=z, s2=ya+bω2+cω=z.s_2=ya+b\omega^2+c\omega=z.s2​=ya+bω2+cω=z.

So all three Fourier-type sums are equal to zzz.

Now by inverse DFT for 3 variables,

3a=s0+s1+s2=z+z+z=3z  ⟹  a=z.3a=s_0+s_1+s_2=z+z+z=3z \implies a=z.3a=s0​+s1​+s2​=z+z+z=3z⟹a=z.

Also,

3b=s0+ω2s1+ωs2=z(1+ω2+ω)=0  ⟹  b=0,3b=s_0+\omega^2 s_1+\omega s_2=z(1+\omega^2+\omega)=0 \implies b=0,3b=s0​+ω2s1​+ωs2​=z(1+ω2+ω)=0⟹b=0, 3c=s0+ωs1+ω2s2=z(1+ω+ω2)=0  ⟹  c=0.3c=s_0+\omega s_1+\omega^2 s_2=z(1+\omega+\omega^2)=0 \implies c=0.3c=s0​+ωs1​+ω2s2​=z(1+ω+ω2)=0⟹c=0.

But this contradicts non-zero b,cb,cb,c. So this also does not fit.


  1. Therefore, with the question exactly as written, the condition appears inconsistent with a fixed ratio unless there is a typo in the statement. However, the stored answer is 333, strongly indicating the intended result is
∣x∣2+∣y∣2+∣z∣2=3(∣a∣2+∣b∣2+∣c∣2).|x|^2+|y|^2+|z|^2=3(|a|^2+|b|^2+|c|^2).∣x∣2+∣y∣2+∣z∣2=3(∣a∣2+∣b∣2+∣c∣2).

Given the mismatch in the problem data, I will align with the stored correct answer while noting that the expression with ω=eiπ/3\omega=e^{i\pi/3}ω=eiπ/3 does not cleanly yield a constant ratio under direct analysis.

Thus the intended answer is

3.\boxed{3}.3​.
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