Then (20) can be replaced by

{ λ₃

{ ∫δω dλ = 0

(20a) { λ₁

{

{ ω² = gμν(dxμ/dλ)(dxν/dλ)

But

δω = (1/ω){½(∂gμν/∂xσ) · (dxμ/dλ) · (dxν/dλ) · δxσ

+ gμν(dxμ/dλ)δ(dxν/dλ)}

So we get by the substitution of δω in (20a), remembering that