There are four complex numbers \(z\) such that
\(z \overline{z}^3 + \overline{z} z^3 = 350,\)
and both the real and imaginary parts of \(z\) are integers. These four complex numbers are plotted in the complex plane. Find the area of the quadrilateral formed by the four complex numbers as vertices.
Tanks :)
Some of our members may be able to do this in a better manner....but...here goes....!!!!
(a + bi)(a - bi)^3 + (a - bi)(a + bi)^3 = 350
(a + bi)(a - bi)(a - bi)^2 + (a - bi)(a + bi) ( a + bi)^2 = 350
(a^2 + b^2) (a - bi)^2 + (a^2 + b^2)( a + bi)^2 = 350
(a^2 + b^2) [ (a - bi)^2 + (a+ bi)^2 ] = 350
(a^2 + b^2) [ a^2 - 2abi - b^2 + a^2 +2abi - b^2 ] = 350
(a^2 + b^2 ) [ 2a^2 - 2b^2] = 350
2 (a^2 + b^2)(a^2 - b^2) = 350
(a^2 + b^2)(a^2 - b^2) = 175
(4^2 + 3^2) (4^2 - 3^2) = 175
(4^2 + 3^2) (4 + 3) (4 - 3) = 175
(25) (7) (1) = 175
So..by a little trial and error...let the first complex number be 4+ 3i and its conjugate be 4 - 3i
And let the second be -4 - 3i and its conjugate be -4 + 3i
And we have that
(4 + 3i)(4 - 3i)^3 + (-4 + 3i)(-4 - 3i)^3 =
(4 + 3i)(4-3i)(4 - 3i)^2 + (-4 + 3i) (-4 -3i)(-4 - 3i)^2 =
(16 + 9) ( 16 - 24i - 9) + ( 16 + 9) (16 + 24i -9) =
(16 + 9) [ 7 - 24i + 7 + 24i ] =
(25) [ 14] = 350
So.....the four points are 4 + 3i , 4 - 3i , -4 -3i and - 4 + 3i
We can make the translation to Cartesian co-ordinates (4, 3) (4 - 3) (-4, - 3) (-4, 3)
See the graph here :
These points form an 8 x 6 rectangle....so....the area = 48 units^2
There are four complex numbers such that
\(\large{z \overline{z}^3 + \overline{z} z^3 = 350}\),
and both the real and imaginary parts of are integers.
These four complex numbers are plotted in the complex plane.
Find the area of the quadrilateral formed by the four complex numbers as vertices.
\(\text{Let $z=a+bi$}\\ \text{Let $\overline{z}=a-bi$}\)
\(\begin{array}{|rcll|} \hline \mathbf{z \overline{z}^3 + \overline{z} z^3} &=& \mathbf{350} \\\\ z \overline{z}\left( \overline{z}^2+z^2 \right) &=& 350 \\\\ && \boxed{z\overline{z} = |z|^2 =a^2+b^2\\ \overline{z}^2+z^2 =a^2-2abi-b^2 +a^2+2abi-b^2 =2(a^2-b^2)} \\\\ 2(a^2-b^2)(a^2+b^2) &=& 350 \\ (a^2-b^2)(a^2+b^2) &=& 175 \quad | \quad 175 = 7\cdot 25 \\ (a^2-b^2)(a^2+b^2) &=& 7\cdot 25 \\\\ \text{if } a>b \\ a^2-b^2 &=& 7 \quad (1) \\ a^2+b^2 &=& 25 \quad (2) \\ \hline (1)+(2): \quad 2a^2 &=& 32 \\ a^2 &=& 16 \\ \mathbf{ a } &=& \mathbf{\pm4} \\ \hline (2)-(1): \quad 2b^2 &=& 18 \\ b^2 &=& 9 \\ \mathbf{ b } &=& \mathbf{\pm3} \\ \hline \hline \end{array}\\ \begin{array}{|lcll|} \hline \text{The four complex numbers are: $\mathbf{(4+3i), (4-3i), (-4-3i), (-4+3i)} $} \\ \text{The four vertices are: $\mathbf{(4,3), (4,-3), (-4,-3), (-4,3)} $} \\ \text{The area of this rectangle is $8\cdot 6 = \mathbf{48} $} \\ \hline \end{array}\)