Bonjour,
a). AD = 3AB
AB ( -1-3 ; 4-5)
AB (-4;-1)
3AB (-4×3;-1×3)
3AB (-12;-3)
donc, AD (-12;-3)
b). AE = 1/3AC+2BC
AC (0-3;2-5)
AC (-3;-3)
1/3AC (-3×1/3;-3×1/3)
1/3 AC = (-1;-1)
BC (0--1;2-4)
BC (1;-2)
2BC (1×2;-2×2)
2BC (2;-4)
AE = 1/3AC+2BC
AE = (-1 +2; -1+-4)
AE (1;-5)
bonne journée ^^