javascript in google spreadsheet is saying that else has a syntax problem... but it doesn't?

if ((Math.cos(x) + 0.5)<=0 || (Math.sin(x))=0);  
  return "not possible to determine the value";

else 

  var a;

var a = (Math.sqrt(Math.cos(x) + 0.5))/(Math.sin(x));
  return a;

}