Soddy's formula

If four circles A, B, C, and D, of radii r₁, r₂, r₃, and r₄, are drawn so that they do not overlap but each touches the other three, and if we let b₁ = 1/r₁, etc., then

(b₁ + b₂ + b₃ + b₄)² = 2(b₁² + b₂² + b₃² + b₄²).