C#: Google map check the value is inside the circle or not..?

hi,
 
 
     I need to find the the latitude langitude value is inside the Radius circle or not.if its is outside i have to pass some value from my MVC Controller. 
 
 




i have tried this but data comes wrong distance,

if the distance is acroos from the radius line i have to show some notification. if anybody knows this please help me for the same

Thanks,

function distance(lat1,lon1,lat2,lon2){
  var R = 6371; // Earth's radius in Km
  return Math.acos(Math.sin(lat1)*Math.sin(lat2) + 
                  Math.cos(lat1)*Math.cos(lat2) *
                  Math.cos(lon2-lon1)) * R;
}


if (distance(user.lat, user.lon, post.lat, post.lon) <= desiredRadiusInKm){
  // return true;
} else {
  // return false;
}