>>498
cos(t) = (a↑・b↑) / |a↑| |b↑|

1)
cos(t) = 5/{ (√5) (√10)} = 1/√2
t = π/4 (45°)

2)
cos(t) = 3/{ 3*2} = 1/2
t = π/3 (60°)

3)
cos(t) = 0
t = π/2 (90°)