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

Sum of four Squares

Euler's four-square identity

Lagrange's four-square theorem - Wikiwand

PDF) Extended Lagrange's four-square theorem

Discrete quantum computation and Lagrange's four-square theorem

Pythagorean Theorem

Lagrange's three square theorem

Stable/Patient Sort: Bit-Manipulation-To-Solve-Problems-Easily-And-Efficiently, PDF, Square Root

Sum of squares, Part-2, Sum of four squares

Waring's Problem -- from Wolfram MathWorld