vector, curve of intersection of x^2 + y^2 = 4 and z = xy

[mathstresser]

Find a vector function that represents the curve of interesection of the two surfaces, x^2 + y^2 = 4 and z = xy.

I know that cosO = (r1×r2) / (|r1||r2|)

but I don't know how to use that.