Same Modulus Complex Roots Problem.

Algebra Level 4

Define a polynomial equation. z 2 + A z + ( 25 + 30 i ) = 125 i z {z^2} + Az + \left( {25 + 30i} \right) = \frac{{125i}}{z}

Let w 1 , w 2 {w_1},{w_2} and w 3 {w_3} be the roots of the equation above. ( w 1 , w 2 , w 3 C {w_1},{w_2},{w_3} \in \mathbb{C} )

If A C A \in \mathbb{C} makes the equation w 1 = w 2 = w 3 \left| {{w_1}} \right| = \left| {{w_2}} \right| = \left| {{w_3}} \right| to be true.

Find the value of w 3 w 3 w 2 w 1 3 ( w 3 ) 1 + w 1 w 1 w 3 w 2 3 ( w 1 ) 1 + w 2 w 2 w 1 w 3 3 ( w 2 ) 1 2 {\left| {\frac{{{w_3} \cdot \overline {{w_3}{w_2}} }}{{{{\left| {{w_1}} \right|}^3}{{\left( {\overline {{w_3}} } \right)}^{ - 1}}}} + \frac{{{w_1} \cdot \overline {{w_1}{w_3}} }}{{{{\left| {{w_2}} \right|}^3}{{\left( {\overline {{w_1}} } \right)}^{ - 1}}}} + \frac{{{w_2} \cdot \overline {{w_2}{w_1}} }}{{{{\left| {{w_3}} \right|}^3}{{\left( {\overline {{w_2}} } \right)}^{ - 1}}}}} \right|^2} .

Additional details

  • Let C \mathbb{C} denotes the set of all complex numbers.
  • i 2 = 1 i^2=-1
61 64 58 67

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1 solution

Kittipong Deevee
Dec 16, 2018

Consider the term. w 3 w 3 w 2 w 1 3 ( w 3 ) 1 \frac{{{w_3} \cdot \overline {{w_3}{w_2}} }}{{{{\left| {{w_1}} \right|}^3}{{\left( {\overline {{w_3}} } \right)}^{ - 1}}}} , As we know that w 1 = w 2 = w 3 \left| {{w_1}} \right| = \left| {{w_2}} \right| = \left| {{w_3}} \right|

  • Change the term. w 3 w 3 w 2 w 1 3 ( w 3 ) 1 \frac{{{w_3} \cdot \overline {{w_3}{w_2}} }}{{{{\left| {{w_1}} \right|}^3}{{\left( {\overline {{w_3}} } \right)}^{ - 1}}}} w 3 w 3 w 3 w 2 w 3 3 \to \frac{{{w_3}\overline {{w_3}} \cdot \overline {{w_3}{w_2}} }}{{{{\left| {{w_3}} \right|}^3}}} , As we know that for any complex number k k , k k = k 2 k\overline k = {\left| k \right|^2}

  • We can change the term through these steps. w 3 w 3 w 3 w 2 w 3 3 w 3 2 w 3 w 2 w 3 3 w 3 w 2 w 3 \frac{{{w_3}\overline {{w_3}} \cdot \overline {{w_3}{w_2}} }}{{{{\left| {{w_3}} \right|}^3}}} \to \frac{{{{\left| {{w_3}} \right|}^2} \cdot \overline {{w_3}{w_2}} }}{{{{\left| {{w_3}} \right|}^3}}} \to \frac{{\overline {{w_3}{w_2}} }}{{\left| {{w_3}} \right|}}

  • Do those steps to the other two and we will get this as the result. w 3 w 3 w 2 w 1 3 ( w 3 ) 1 + w 1 w 1 w 3 w 2 3 ( w 1 ) 1 + w 2 w 2 w 1 w 3 3 ( w 2 ) 1 2 = w 3 w 2 w 3 + w 1 w 3 w 1 + w 2 w 1 w 2 2 {\left| {\frac{{{w_3} \cdot \overline {{w_3}{w_2}} }}{{{{\left| {{w_1}} \right|}^3}{{\left( {\overline {{w_3}} } \right)}^{ - 1}}}} + \frac{{{w_1} \cdot \overline {{w_1}{w_3}} }}{{{{\left| {{w_2}} \right|}^3}{{\left( {\overline {{w_1}} } \right)}^{ - 1}}}} + \frac{{{w_2} \cdot \overline {{w_2}{w_1}} }}{{{{\left| {{w_3}} \right|}^3}{{\left( {\overline {{w_2}} } \right)}^{ - 1}}}}} \right|^2} = {\left| {\frac{{\overline {{w_3}{w_2}} }}{{\left| {{w_3}} \right|}} + \frac{{\overline {{w_1}{w_3}} }}{{\left| {{w_1}} \right|}} + \frac{{\overline {{w_2}{w_1}} }}{{\left| {{w_2}} \right|}}} \right|^2}

  • Again, consider the fact that w 1 = w 2 = w 3 \left| {{w_1}} \right| = \left| {{w_2}} \right| = \left| {{w_3}} \right| w 3 w 2 w 3 + w 1 w 3 w 1 + w 2 w 1 w 2 2 w 3 w 2 w 1 + w 1 w 3 w 1 + w 2 w 1 w 1 2 w 3 w 2 + w 1 w 3 + w 2 w 1 w 1 2 w 3 w 2 + w 1 w 3 + w 2 w 1 2 w 1 2 {\left| {\frac{{\overline {{w_3}{w_2}} }}{{\left| {{w_3}} \right|}} + \frac{{\overline {{w_1}{w_3}} }}{{\left| {{w_1}} \right|}} + \frac{{\overline {{w_2}{w_1}} }}{{\left| {{w_2}} \right|}}} \right|^2} \to {\left| {\frac{{\overline {{w_3}{w_2}} }}{{\left| {{w_1}} \right|}} + \frac{{\overline {{w_1}{w_3}} }}{{\left| {{w_1}} \right|}} + \frac{{\overline {{w_2}{w_1}} }}{{\left| {{w_1}} \right|}}} \right|^2} \to {\left| {\frac{{\overline {{w_3}{w_2}} + \overline {{w_1}{w_3}} + \overline {{w_2}{w_1}} }}{{\left| {{w_1}} \right|}}} \right|^2} \to \frac{{{{\left| {\overline {{w_3}{w_2}} + \overline {{w_1}{w_3}} + \overline {{w_2}{w_1}} } \right|}^2}}}{{{{\left| {{w_1}} \right|}^2}}}

From the polynomial. z 2 + A z + ( 25 + 30 i ) = 125 i z {z^2} + Az + \left( {25 + 30i} \right) = \frac{{125i}}{z} , As we know that z 0 z \ne 0 .

  • Multiply by z z both sides. z 2 + A z + ( 25 + 30 i ) = 125 i z z 3 + A z 2 + ( 25 + 30 i ) z 125 i = 0 {z^2} + Az + \left( {25 + 30i} \right) = \frac{{125i}}{z} \to {z^3} + A{z^2} + \left( {25 + 30i} \right)z - 125i = 0

  • From the Vieta's formular. We know that. w 1 w 2 + w 2 w 3 + w 3 w 1 = 25 + 30 i w 1 w 2 w 3 = 125 i \begin{array}{c} {w_1}{w_2} + {w_2}{w_3} + {w_3}{w_1} = 25 + 30i\\ {w_1}{w_2}{w_3} = 125i \end{array}

  • Take the absolute both sides. w 1 w 2 + w 2 w 3 + w 3 w 1 = 25 + 30 i w 1 w 2 + w 2 w 3 + w 3 w 1 2 = 1525 a n d w 1 w 2 w 3 = 125 i w 1 3 = 125 w 1 2 = 25 \begin{array}{c} \left| {{w_1}{w_2} + {w_2}{w_3} + {w_3}{w_1}} \right| = \left| {25 + 30i} \right| \to {\left| {{w_1}{w_2} + {w_2}{w_3} + {w_3}{w_1}} \right|^2} = 1525\\ {\rm{and}}\\ \left| {{w_1}{w_2}{w_3}} \right| = \left| {125i} \right| \to {\left| {{w_1}} \right|^3} = 125 \to {\left| {{w_1}} \right|^2} = 25 \end{array}

  • Plug in the value. w 3 w 2 + w 1 w 3 + w 2 w 1 2 w 1 2 = w 1 w 2 + w 2 w 3 + w 3 w 1 2 w 1 2 = 1525 25 = 61 \frac{{{{\left| {\overline {{w_3}{w_2}} + \overline {{w_1}{w_3}} + \overline {{w_2}{w_1}} } \right|}^2}}}{{{{\left| {{w_1}} \right|}^2}}} = \frac{{{{\left| {{w_1}{w_2} + {w_2}{w_3} + {w_3}{w_1}} \right|}^2}}}{{{{\left| {{w_1}} \right|}^2}}} = \frac{{1525}}{{25}} = 61

So, w 3 w 3 w 2 w 1 3 ( w 3 ) 1 + w 1 w 1 w 3 w 2 3 ( w 1 ) 1 + w 2 w 2 w 1 w 3 3 ( w 2 ) 1 2 = 61 {\left| {\frac{{{w_3} \cdot \overline {{w_3}{w_2}} }}{{{{\left| {{w_1}} \right|}^3}{{\left( {\overline {{w_3}} } \right)}^{ - 1}}}} + \frac{{{w_1} \cdot \overline {{w_1}{w_3}} }}{{{{\left| {{w_2}} \right|}^3}{{\left( {\overline {{w_1}} } \right)}^{ - 1}}}} + \frac{{{w_2} \cdot \overline {{w_2}{w_1}} }}{{{{\left| {{w_3}} \right|}^3}{{\left( {\overline {{w_2}} } \right)}^{ - 1}}}}} \right|^2} = 61

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