The angle between the vectors a + b and a - b when a = (1, 1, 4) and b = (1, - 1, 4) is
45°
90°
15°
30°
If ABCDEF is a regular hexagon, then AD + EB + FC equals
0
2 AB
4 AB
3 AB
C.
4 AB
Let AB = a, BC = b, CD = c
AD = a + b + c
EB = ED + DC + CB = a - c - b
FC = FA + AB + BC = - c + a + b