Complex Rhombus

Geometry Level 5

The equation 2 z 4 + 8 i z 3 + ( 11 + 7 i ) z 2 + ( 14 6 i ) z + 25 14 i = 0 2z^4 + 8i z^3 + (-11+7i) z^2 + (-14-6i) z + 25-14i = 0 has 4 complex roots, which form a rhombus on the complex plane. The area of this rhombus can be expressed as A \sqrt{A} , find A A .


The answer is 50.

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1 solution

Albert Yiyi
Jul 13, 2018

  1. let f ( z ) = 2 z 4 + 8 i z 3 + ( 11 + 7 i ) z 2 + ( 14 6 i ) z + 25 14 i = 2 ( z z 1 ) ( z z 2 ) ( z z 3 ) ( z z 4 ) f(z) = 2z^4 + 8i z^3 + (-11+7i) z^2 + (-14-6i) z + 25-14i = 2(z-z_1)(z-z_2)(z-z_3)(z-z_4) where z k z_k are the roots.

  2. sum of roots = 8 i 2 = 4 i = \frac{-8i}{2} = -4i , average of roots = 4 i 4 = i = \frac{-4i}{4} = -i which gives the center of the rhombus.

  3. a 2 b 2 = ( i z 1 ) ( i z 2 ) ( i z 3 ) ( i z 4 ) = 1 2 f ( i ) = 12 7 2 i = 25 2 a b = 25 2 a^2 b^2 = |(-i-z_1)(-i-z_2)(-i-z_3)(-i-z_4)| = |\frac{1}{2}f(-i)| = |12 - \frac{7}{2}i| = \frac{25}{2} \implies ab = \sqrt{\frac{25}{2}}

  4. area of rhombus = 2 a b = 50 A = 50 = 2ab = \sqrt{50} \implies \therefore A=50

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