Sum of Two Fourth Powers
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Theorem
- $x^4 + y^4 = \paren {x^2 + \sqrt 2 x y + y^2} \paren {x^2 - \sqrt 2 x y + y^2}$
Proof
\(\ds \) | \(\) | \(\ds \paren {x^2 + \sqrt 2 x y + y^2} \paren {x^2 - \sqrt 2 x y + y^2}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds x^2 \paren {x^2 - \sqrt 2 x y + y^2} + \sqrt 2 x y \paren {x^2 - \sqrt 2 x y + y^2} + y^2 \paren {x^2 - \sqrt 2 x y + y^2}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds x^4 - \sqrt 2 x^3 y + x^2 y^2 + \sqrt 2 x^3 y - 2 x^2 y^2 + \sqrt 2 x y^3 + x^2 y^2 - \sqrt 2 x y^3 + y^4\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds x^4 + y^4\) | gathering terms |
$\blacksquare$