Formula for the angle between two Vectors
One could do better than guess by noticing that in going from the
tail to the head of a the vertical distance increases by 4
while the horizontal distance increases by 4 
2.
Hence the tangent of the angle is
4/(4 2)
= 1.0/2
= 0.7071,
so the angle with the horizontal is arctan( 0.7071 ) = 35.26°.
Since b is in the horizontal plane,
the angle between the two vectors must be 35°.
The formula for the angle θ between two unit vectors is:
au · bu = cos θ
To use this formula with non-unit vectors: (1) normalize each vector, (2) compute the dot product, (3) take the arc cos to get the angle.
