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Let zˉ denote the complex conjugate of a complex number z. If z is a non-zero complex number for which both real and imaginary parts of (zˉ)2+z21 are integers, then which of the following is/are possible value(s) of ∣z∣ ?
A
(243+3205)41
B
(47+33)41
C
(49+65)41
D
(67+13)41
View written solutionFree
Correct answer: A
Step-by-step Solution
Let the given complex number be w=(zˉ)2+z21. We are given that the real and imaginary parts of w are integers. Let Re(w)=k and Im(w)=m, where k,m∈Z.
Let's express z in its polar form, z=reiθ, where r=∣z∣ is the modulus and θ is the argument. The complex conjugate is zˉ=re−iθ.
Now, we substitute this into the expression for w:
(zˉ)2=(re−iθ)2=r2e−i2θz2=(reiθ)2=r2ei2θz21=r2ei2θ1=r21e−i2θ
So, the expression for w becomes:
w=r2e−i2θ+r21e−i2θ=(r2+r21)e−i2θ
Using Euler's formula, e−i2θ=cos(2θ)−isin(2θ), we can write w in Cartesian form:
w=(r2+r21)(cos(2θ)−isin(2θ))
From this, we can identify the real and imaginary parts of w:
Re(w)=(r2+r21)cos(2θ)=k (Equation 1)
Im(w)=−(r2+r21)sin(2θ)=m (Equation 2)
To eliminate θ, we can square both equations and add them. This uses the trigonometric identity cos2(2θ)+sin2(2θ)=1.
k2=(r2+r21)2cos2(2θ)m2=(r2+r21)2sin2(2θ)
Adding these two gives:
k2+m2=(r2+r21)2(cos2(2θ)+sin2(2θ))k2+m2=(r2+r21)2
Let's expand the right side and express it in terms of R=∣z∣4=r4.
k2+m2=(r2)2+2(r2)(r21)+(r21)2=r4+2+r41k2+m2=R+2+R1
This gives us a necessary condition on R=∣z∣4:
R+R1=k2+m2−2
Since k and m are integers, k2+m2−2 must be an integer. Therefore, for a value of ∣z∣ to be possible, ∣z∣4+∣z∣41 must be an integer.
Now we test each option by calculating R=∣z∣4 and checking if R+R1 is an integer.
A: ∣z∣=(243+3205)41R=∣z∣4=243+3205.
R1=43+32052=(43)2−(3205)22(43−3205)=1849−9(205)2(43−3205)=1849−18452(43−3205)=42(43−3205)=243−3205.
R+R1=243+3205+243−3205=286=43.
Since 43 is an integer, this option is potentially correct. We need to check if k2+m2−2=43, which means k2+m2=45. The number 45 can be written as a sum of two squares, for example, 45=62+32. So we can have integers k=6,m=3. Thus, this option is possible.
B: ∣z∣=(47+33)41R=∣z∣4=47+33.
R1=7+334=49−334(7−33)=164(7−33)=47−33.
R+R1=47+33+47−33=414=27.
This is not an integer. Therefore, this option is not possible.
C: ∣z∣=(49+65)41R=∣z∣4=49+65.
R1=9+654=81−654(9−65)=164(9−65)=49−65.
R+R1=49+65+49−65=418=29.
This is not an integer. Therefore, this option is not possible.
D: ∣z∣=(67+13)41R=∣z∣4=67+13.
R1=7+136=49−136(7−13)=366(7−13)=67−13.
R+R1=67+13+67−13=614=37.
This is not an integer. Therefore, this option is not possible.
Only option A satisfies the necessary condition. Therefore, it is the only possible value for ∣z∣ among the given choices.