petite, et que l’on regarde les deux points
et
comme appartenant à la tangente à la corde, menée par le point
il en résultera
![{\displaystyle {\frac {x'}{\sigma }}={\frac {dx}{ds}},\qquad {\frac {y'}{\sigma }}={\frac {dy}{ds}},\qquad {\frac {z'}{\sigma }}={\frac {dz}{ds}}\,;}](https://wikimedia.org/api/rest_v1/media/math/render/svg/7e2dd2ab6d1ef48b5268d87324f85c6cba58a0d7)
par conséquent nous aurons
![{\displaystyle {\begin{aligned}\delta &={\frac {du}{dx}}{\frac {dx^{2}}{ds^{2}}}+{\frac {dv}{dy}}{\frac {dy^{2}}{ds^{2}}}+{\frac {dw}{dz}}{\frac {dz^{2}}{ds^{2}}}\\&+\left({\frac {du}{dy}}+{\frac {dv}{dx}}\right){\frac {dxdy}{ds^{2}}}++\left({\frac {du}{dz}}+{\frac {dw}{dx}}\right){\frac {dxdz}{ds^{2}}}++\left({\frac {dv}{dz}}+{\frac {dw}{dy}}\right){\frac {dydz}{ds^{2}}}.\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/35c245d5f07d1aaa4e261ca843762242b4375439)
Mais d’après les formules du no 7, et les résultats du no 23, nous avons
![{\displaystyle {\begin{aligned}\mathrm {P_{2}=T} {\frac {dydx}{ds^{2}}}&=-k\left({\frac {du}{dy}}+{\frac {dv}{dx}}\right),\\\mathrm {P_{1}=T} {\frac {dxdz}{ds^{2}}}&=-k\left({\frac {du}{dz}}+{\frac {dw}{dx}}\right),\\\mathrm {Q_{1}=T} {\frac {dydz}{ds^{2}}}&=-k\left({\frac {dv}{dz}}+{\frac {dw}{dy}}\right)\,;\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e7a7c16bdd9783d13c914dc5806254b2617103c8)
ce qui change d’abord la formule précédente en celle-ci :
![{\displaystyle k\delta =k\left({\frac {du}{dx}}{\frac {dx^{2}}{ds^{2}}}+{\frac {dv}{dy}}{\frac {dy^{2}}{ds^{2}}}+{\frac {dw}{dz}}{\frac {dz^{2}}{ds^{2}}}\right)-\mathrm {T} \left({\frac {dx^{2}dy^{2}+dx^{2}dz^{2}+dy^{2}dz^{2}}{ds^{2}}}\right).}](https://wikimedia.org/api/rest_v1/media/math/render/svg/fc10cfb934f255506b6226a33f25fe4f0aecf285)
Nous avons de plus
![{\displaystyle {\begin{alignedat}{2}\mathrm {P} _{3}&=\mathrm {T} {\frac {dx^{2}}{ds^{2}}}&&=-k\left(3{\frac {du}{dx}}+{\frac {dv}{dy}}+{\frac {dw}{dz}}\right),\\\mathrm {Q} _{2}&=\mathrm {T} {\frac {dy^{2}}{ds^{2}}}&&=-k\left({\frac {du}{dx}}+3{\frac {dv}{dy}}+{\frac {dw}{dz}}\right),\\\mathrm {R} _{1}&=\mathrm {T} {\frac {dz^{2}}{ds^{2}}}&&=-k\left({\frac {du}{dx}}+{\frac {dv}{dy}}+3{\frac {dw}{dz}}\right)\,;\end{alignedat}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/eb86de15590dcaf2bfcd6d846d6c8e0c0a9e32fb)
d’où je tire les valeurs de
pour les substituer