"Doctor61" wrote in message <email@example.com>... > I have two 3d circles (centre coordiantes and radii) with their normals passing through origin. If you consider the origin to be the vertex of a right cone having the circle as the base, how can I determine if there is an intersection between these cones?
if centre is x1,y1,z1 and radius r set d^2=x1^2+y1^2+z1^2 Cone is intersection of spheres a^2 (d^2+r^2)=x^2+y^2+z^2 and a^2 r^2 = (a x1-x)^2+(a y1-y)^2+(a z1-z)^2
Eliminate a to get equation of cone: x^2+y^2+z^2 = (r^2+d^2) (x1 x+y1 y+z1 z)^2/d^4 (check this)
Set x^2+y^2+z^2=1 and solve for points on two cones and this sphere. E.g. Solve linear equations (1= (r^2+d^2) (x1 x+y1 y+z1 z)^2/d^4 etc) for x and y then substitute into x^2+y^2+z^2=1 to get quadratic for z.