Complex Modulus/Examples/3iz - z^2
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Example of Complex Modulus
Let:
- $w = 3 i z - z^2$
where $z = x + i y$.
Then:
- $\cmod w^2 = x^4 + y^4 + 2 x^2 y^2 - 6 x^2 y - 6 y^3 + 9 x^2 + 9 y^2$
Proof
\(\ds \cmod w^2\) | \(=\) | \(\ds \cmod {3 i z - z^2}^2\) | Definition of $w$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \cmod {3 i \paren {x + i y} - \paren {x + i y}^2}^2\) | Definition of $z$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \cmod {3 i x - 3 y - x^2 + y^2 - 2 i x y}^2\) | multiplying out | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {-x^2 + y^2 - 3 y}^2 + \paren {3 x - 2 x y}^2\) | Definition of Complex Modulus | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {x^4 - 2 x^2 y^2 + 6 x^2 y + y^4 - 6 y^3 + 9 y^2} + \paren {9 x^2 - 12 x^2 y + 4 x^2 y^2}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds x^4 + y^4 - 2 x^2 y^2 + 4 x^2 y^2 + 6 x^2 y - 12 x^2 y - 6 y^3 + 9 x^2 + 9 y^2\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds x^4 + y^4 + 2 x^2 y^2 - 6 x^2 y - 6 y^3 + 9 x^2 + 9 y^2\) |
$\blacksquare$
Sources
- 1981: Murray R. Spiegel: Theory and Problems of Complex Variables (SI ed.) ... (previous) ... (next): $1$: Complex Numbers: Supplementary Problems: Fundamental Operations with Complex Numbers: $60$