title: Angle between 2 Lines in 3D
body: <p>i know how to get Angles with atan2 between 2 Points in 2D, but how does this work in 3D?:
  lets say i have 3 Points A,B,C (all are SCNVector3 with x,y,z coordinates
  First Line Endpoints A and B
  2nd Line Endpoints B and C
  Now i want to get the angle between the 2 Lines... (in ios Swift)
  I read something about the dot product and acos but somehow it does not work...</p>
  
  <p>With i=0:</p>
  
  <pre><code>        var vector1 = SCNVector3((pointArray[i].x - pointArray[i+1].x), (pointArray[i].y - pointArray[i+1].y), (pointArray[i].z - pointArray[i+1].z))
          var vector2 = SCNVector3((pointArray[i+2].x - pointArray[i+1].x), (pointArray[i+2].y - pointArray[i+1].y), (pointArray[i+2].z - pointArray[i+1].z))
          var dotProduct = vector1.x * vector2.x + vector1.y * vector2.y + vector1.z * vector2.z
          var theta = acos(dotProduct)
          var tmp_winkel = GLKMathRadiansToDegrees(theta)
  </code></pre>