title: what is the geometry meaning of 2D cross product?
body: <p>I have two 2D vectors, like [x1,y1] and [x2,y2]</p>
  
  <p>Some guys define 2d Cross as x1*y2 - y1 * x2</p>
  
  <p>I am wondering what's the meaning of this? Any practical application?</p>