title: Map vector onto plane and get angle between it and other plane intersection
body: <p>I'm making a game using the Unity3D game engine and C#.</p>
  
  <p>In my game I have two 3d vectors, A-B and C-D. I'd like to determine the angle E but I dont know enough geometry and math to work out how.
  <img src="https://i.stack.imgur.com/XA1Sf.png" alt="enter image description here"></p>
  
  <p>What I want to do is..
  Project the vector C-D onto a plane thats normal to A-B .
  Then measure the angle on the plane between this projected vector and another vector that represents the intersection between the plane and another plane which has a completely flat orientation.</p>
  
  <p>In your answer I'd appreciate it if you wrote the equations as if they were code (ie. this variable multiplied by this one divided by this etc.) and not mathematical notation which I dont know.</p>