13.2 On utilise les conventions usuelles~a=
a1 a2 a3
et~b=
b1 b2 b3
.
1) (~a×~b)·~a=
a2b3−a3b2 a3b1−a1b3 a1b2−a2b1
·
a1 a2 a3
=
(a2b3−a3b2)a1+ (a3b1 −a1b3)a2+ (a1b2−a2b1)a3 = a1a2b3−a1a3b2+a2a3b1−a1a2b3+a1a3b2 −a2a3b1 = 0
2) (~a×~b)·~b=
a2b3−a3b2 a3b1−a1b3 a1b2−a2b1
·
b1 b2 b3
=
(a2b3−a3b2)b1+ (a3b1−a1b3)b2+ (a1b2−a2b1)b3 = a2b1b3−a3b1b2+a3b1b2−a1b2b3+a1b2b3−a2b1b3 = 0
Géométrie : produit vectoriel Corrigé 13.2