Three dimensional geometry

• Find the Angle Between Two Planes from Three Points
  Use a cross product to find the second plane’s normal, then compare it with the normal to z = 0 to get the acute angle.
• How to Parametrize a Circle in 3D from Its Center and Two Points
  Given a center and two points, form a radius vector and an orthonormal basis for their plane, then parametrize the circle with sine and cosine.