Differential of Sum of Squares

Theorem

$\map \d {x^2 + y^2} = 2 \paren {x \map \rd x + y \map \rd y}$

Proof

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Sources

• 1972: George F. Simmons: Differential Equations ... (previous) ... (next): $\S 2.9$: Integrating Factors: $(14)$