2. Corollaire I. — Donc, si l’on a entre les neuf quantités précédentes ces six équations
![{\displaystyle {\begin{alignedat}{2}x^{2}\,\ +y^{2}\ \ +z^{2}\ =&a,&x'x''+y'y''+z'z''=&b\\x'^{2}\,+y'^{2}\ +z'^{2}\,=&a',&x\,x''\ +yy''\ +zz''\ =&b',\\x''^{2}+y''^{2}+z''^{2}=&a'',\qquad &xx'\ \ +yy'\ \ +zz'\ \ =&b'',\end{alignedat}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/4b4de1148ea72ecf18a11d7199d2aad65ec918d8)
et qu’on fasse, pour abréger,
![{\displaystyle \xi =y'z''-z'y'',\quad \eta =z'x''-x'z'',\quad \zeta =x'y''-y'x'',}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e8aa8480c4264b2245ca99d44d695d76ae06fc9b)
![{\displaystyle \beta ={\sqrt {aa'a''+2bb'b''-ab^{2}-a'b'^{2}-a''b''^{2}}},}](https://wikimedia.org/api/rest_v1/media/math/render/svg/7f6846c04cb39a545217445a078887e4a82d613e)
on aura
![{\displaystyle x\xi +y\eta +z\zeta =\beta .}](https://wikimedia.org/api/rest_v1/media/math/render/svg/11588821250620f1a3c17791d7b1df04af15c871)
On aura de plus les équations identiques suivantes
![{\displaystyle x'\xi +y'\eta +z'\zeta =0,\quad x''\xi +y''\eta +z''\zeta =0,\quad \xi ^{2}+\eta ^{2}+\zeta ^{2}=a'a''-b^{2},}](https://wikimedia.org/api/rest_v1/media/math/render/svg/bc8fd683218757aaabc699c540fe4adfc46bf256)
![{\displaystyle {\begin{alignedat}{2}y'\zeta -z'\eta =&bx'-a'x'',\qquad &y''\zeta -z''\eta =&a''x'-bx'',\\z'\xi -x'\zeta =&by'-a'y'',&z''\xi -x''\zeta =&a''y'-by'',\\x'\eta -y'\xi =&bz'-a'z'',&x''\eta -y''\xi =&a''z'-bz'',\end{alignedat}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e607945f36a2010add439edd29c380b2cf29911c)
qui sont très-faciles à vérifier par le calcul.
3. Corollaire II. Si l’on prend les trois équations
![{\displaystyle {\begin{aligned}x\xi \,\ \ +y\eta \ \ +z\zeta \ \ =&\beta ,\\xx'\,+yy'\ +zz'\ =&b'',\\xx''+yy''+zz''=&b',\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/3ab5b6d7af22ff7c017ca8911cd2cc234364ebdb)
et qu’on en tire les valeurs des quantités
on aura par les formules connues
![{\displaystyle {\begin{aligned}x=&{\frac {\beta (y'z''-z'y'')+b'(\eta z'-\zeta y')+b''(\zeta y''-\eta z'')}{\xi (y'z''-z'y'')+\eta (z'x''-x'z'')+\zeta (x'y''-y'x'')}},\\\\y=&{\frac {\beta (z'x''-x'z'')+b'(\zeta x'-\xi z')+b''(\xi z''-\zeta x'')}{\xi (y'z''-z'y'')+\eta (z'x''-x'z'')+\zeta (x'y''-y'x'')}},\\\\z=&{\frac {\beta (x'y''-y'x'')+b'(\xi y'-\eta x')+b''(\eta x''-\xi y'')}{\xi (y'z''-z'y'')+\eta (z'x''-x'z'')+\zeta (x'y''-y'x'')}}\,;\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8a471fcfc9227e17320a80a7d658e5764e792524)