Des formules

sin(^i -{- b) = sinacos^ -+- cosasin^, cos(a 4- b) =cosacosè — sinasin^, sin(a — b) = sinacos^ — cosasîn^, cos(a — b) =icosacosb H- sinasin^, on déduit

isiu(a -h b) -h sm(a — b) = 2sinacosb, sm[a -\- b) — sin(fl — b)z= icosa sine, cos(û-4- b) -h cos(a — b) = 'zcosacos^, cos(a — b) — cos(a + b) = asina siab.

Soient maintenant

a-^- b =p, a — bz=.q\ ,

on aura