donc, faisant maintenant
on aura
![{\displaystyle \mathrm {X} =0\quad {\text{et}}\quad {\frac {d\mathrm {X} }{dx}}-{\frac {p}{u}}{\frac {d\mathrm {X} }{du}}=\mathrm {D} .}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c506fc24b5e9c412ce66f9217ded8699744afa05)
17. Cela posé, si l’on considère
comme une fonction de
et
et qu’on suppose
![{\displaystyle d\mathrm {X} =\mathrm {P} dx+\mathrm {Q} da,\quad d\mathrm {P} =\mathrm {R} dx+\mathrm {S} da,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d7f12918ebd44976fea2812024b961eb39575a1f)
on aura
![{\displaystyle {\frac {d\mathrm {X} }{dx}}=\mathrm {P} \quad {\text{et}}\quad {\frac {d^{2}\mathrm {X} }{dx^{2}}}=\mathrm {R} ,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/640bf432f56acb9d8f5988a4877b55f4cf671e91)
de sorte que la quantité
deviendra
![{\displaystyle q=\mathrm {X} {\frac {dp}{dx}}+\mathrm {P} \left(u{\frac {dp}{du}}-p\right)+\mathrm {R} u^{2}.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/01d1cbe6877d4b67e265b405fa0418a63ec41d72)
Regardons maintenant la quantité
comme une fonction de
et
et l’on aura aussi
![{\displaystyle {\begin{aligned}d\mathrm {X} =&\mathrm {T} dx\ +\mathrm {V} du,\\d\mathrm {T} =&\mathrm {W} dx+\mathrm {Y} du,\\d\mathrm {V} =&\mathrm {Y} dx\ +\mathrm {Z} du\,;\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/ff9cc65d61c601b3fa97051d830e26dceefdbc19)
or on a, par l’équation (E),
![{\displaystyle du=-{\frac {pdx}{u}}-{\frac {rda}{u}},}](https://wikimedia.org/api/rest_v1/media/math/render/svg/90013a0c0ad906fa9c2e27f49d41dac980977961)
donc
![{\displaystyle d\mathrm {X} =\left(\mathrm {T} -{\frac {p\mathrm {V} }{u}}\right)dx-{\frac {r\mathrm {V} }{u}}du\,;}](https://wikimedia.org/api/rest_v1/media/math/render/svg/794567926a09140e62971f2335751fa2a5ac161f)
donc
![{\displaystyle \mathrm {P=T} -{\frac {p\mathrm {V} }{u}}\,;}](https://wikimedia.org/api/rest_v1/media/math/render/svg/f1c33592f129d9846423c49628b78efa96493987)
différentions maintenant cette valeur de
et l’on aura
![{\displaystyle d\mathrm {P} =\mathrm {W} dx+\mathrm {Y} du-\mathrm {V} d{\frac {p}{u}}-{\frac {p}{u}}(\mathrm {Y} dx+\mathrm {Z} du),}](https://wikimedia.org/api/rest_v1/media/math/render/svg/ca1de6a0ac593e5ce245dd37d346730217b4a803)
c’est-à-dire
![{\displaystyle d\mathrm {P} =\left(\mathrm {W} -{\frac {\mathrm {V} }{u}}{\frac {dp}{dx}}-\mathrm {Y} {\frac {p}{u}}\right)+\left(\mathrm {Y} -{\frac {\mathrm {V} }{u}}{\frac {dp}{du}}+{\frac {\mathrm {V} p}{u^{3}}}-{\frac {p\mathrm {Z} }{u}}\right)du,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/ab1c5546ccb515c65fd36620e5f9218bb2f86b37)