In reply to Larry Kemp.

Yes. If u and v are two unit vectors, then the angle between them (theta) satisfies the equation u*v = cos(theta).
Therefore you can invert the equation to find theta. In the SAS/IML language I would use
proc iml;
u = {1 1};
v = {-1 -2};
w = u*v` / (norm(u)*norm(v));
theta = arcos( w );
deg = 180/constant("pi") * theta;
print theta deg;