code: function ( x1, y1, x2, y2, cx, cy ) {
          var a = Math.sqrt( Math.pow(x1 - x2, 2) + Math.pow(y1 - y2, 2) ),
              b = Math.sqrt( Math.pow(x1 - cx, 2) + Math.pow(y1 - cy, 2) ),
              c = Math.sqrt( Math.pow(cx - x2, 2) + Math.pow(cy - y2, 2) ),
              r = ( Math.pow(b,2) + Math.pow(c,2) - Math.pow(a,2) ) / ( 2 * b * c ),
              radian = Math.acos( r );
  
          //return radian * ( 180 / Math.PI );
          return this._radianToDeg( radian );
      }
docs: 3점의 각도 구하기 (기준점 cx, cy)