Why was this visual proof missed for 400 years? (Fermat's two square theorem)

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Math & Physics - Lagrange's Four-square Theorem Follow instagram.com/ins.map.lb

Jacobi's four-square theorem

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Lagrange's Four-Square Theorem Seen Using Polygons and Lines

Waring's Problem & Lagrange's Four-Square Theorem

CodeChef - Lagrange's four-square theorem, also known as Bachet's conjecture, states that every natural number can be represented as the sum of four integer squares. This theorem was proven by Joseph Louis

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Waring's Problem -- from Wolfram MathWorld