changeant dans cette expression
en
et
en
on a pour l’intégrale totale, à cause que
![{\displaystyle {\frac {1}{a}}\left(\pi -\varepsilon -\operatorname {arc.tang} {\frac {a(s'-s\cos .\varepsilon )}{s\sin .\varepsilon {\sqrt {a^{2}+s^{2}+s'^{2}-2ss'\cos .\varepsilon }}}}-\operatorname {arc.tang} .{\frac {a\cot .\varepsilon }{\sqrt {a^{2}+s^{2}}}}\right.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/4779f1f835987d3fd8659d9a20a6dedd4db3869b)
![{\displaystyle \left.-\operatorname {arc.tang} {\frac {a(s-s'\cos .\varepsilon )}{s'\sin .\varepsilon {\sqrt {a^{2}+s^{2}+s'^{2}-2ss'\cos .\varepsilon }}}}-\operatorname {arc.tang} .{\frac {a\cot .\varepsilon }{\sqrt {a^{2}+s'^{2}}}}\right).}](https://wikimedia.org/api/rest_v1/media/math/render/svg/db6f9a040bd10550b0ffdf07ebd07ecaa2dec457)
En calculant la tangente de la somme des deux arcs dont les valeurs contiennent
et
on change cette expression en
![{\displaystyle {\frac {1}{a}}\left(\pi -\varepsilon -\operatorname {arc.tang} {\frac {a\sin .\varepsilon {\sqrt {a^{2}+s^{2}+s'^{2}-2ss'\cos .\varepsilon }}}{ss'\sin .^{2}\varepsilon +a^{2}\cos .\varepsilon }}\right.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/986a5ca0a63afb74e40077e7ac0f6db92ec06891)
![{\displaystyle \left.-\operatorname {arc.tang} {\frac {a\cot .\varepsilon }{\sqrt {a^{2}+s^{2}}}}-\operatorname {arc.tang} .{\frac {a\cot .\varepsilon }{\sqrt {a^{2}+s'^{2}}}}\right)\,;}](https://wikimedia.org/api/rest_v1/media/math/render/svg/5e856b0c40461ddbc959a57cd18a169db24d99d0)
et comme
![{\displaystyle {\frac {\pi }{2}}-\operatorname {arc.tang} {\frac {a\sin .\varepsilon {\sqrt {a^{2}+s^{2}+s'^{2}-2ss'\cos .\varepsilon }}}{ss'\sin .^{2}\varepsilon +a^{2}\cos .\varepsilon }}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d57e6397939bd3c409a88675e3aa4310cea7d229)
![{\displaystyle =\operatorname {arc.tang} {\frac {ss'\sin .^{2}\varepsilon +a^{2}\cos .\varepsilon }{a\sin .\varepsilon {\sqrt {a^{2}+s^{2}+s'^{2}-2ss'\cos .\varepsilon }}}},}](https://wikimedia.org/api/rest_v1/media/math/render/svg/77ce60addeab2d0296ae9063b19216e95a4a5c87)
on a, en divisant par ![{\displaystyle \sin .\varepsilon ,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/a1b86ceea4b1fd88d03e684236a4d53d5e7cb2ca)
![{\displaystyle \iint {\frac {\mathrm {d} s\mathrm {d} s'}{r^{3}}}={\frac {1}{a\sin .\varepsilon }}\left(\operatorname {arc.tang} {\frac {ss'\sin .^{2}\varepsilon +a^{2}\cos .\varepsilon }{a\sin .\varepsilon {\sqrt {a^{2}+s^{2}+s'^{2}-2ss'\cos .\varepsilon }}}}\right.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/4854bb99aaa0cbddcc693c2e708fb0dae1a2b378)
![{\displaystyle \left.-\operatorname {arc.tang} {\frac {a\cot .\varepsilon }{\sqrt {a^{2}+s^{2}}}}-\operatorname {arc.tang} .{\frac {a\cot .\varepsilon }{\sqrt {a^{2}+s'^{2}}}}+{\frac {\pi }{2}}-\varepsilon \right)\,;}](https://wikimedia.org/api/rest_v1/media/math/render/svg/57143bc86438be48b3d8a24b5502bca104ad8bc8)
expression qui, lorsqu’on suppose
se réduit à
![{\displaystyle {\frac {1}{a}}\left(\operatorname {arc.tang} {\frac {ss'}{a{\sqrt {a^{2}+s^{2}+s'^{2}}}}}\right),}](https://wikimedia.org/api/rest_v1/media/math/render/svg/311bb88a20893d4a25dc7008e5d72ee5b98af553)
comme nous l’avons trouvé précédemment.