@article{NAM_1865_2_4__421_0,
     author = {Barbier, E.},
     title = {D\'emonstration g\'eom\'etrique des formules de trigonom\'etrie sph\'erique qui donnent
$\sin \frac{A}{2}, \cos \frac{A}{2}, \mathrm {tang}\frac{A}{2}$},
     journal = {Nouvelles annales de math\'ematiques : journal des candidats aux \'ecoles polytechnique et normale},
     pages = {421--423},
     publisher = {Carilian-Goeury et Vor Dalmont},
     volume = {2e s{\'e}rie, 4},
     year = {1865},
     language = {fr},
     url = {http://www.numdam.org/item/NAM_1865_2_4__421_0/}
}