Check if a latitude and longitude is within a circle

Question

See this illustration:

What I would like to know is:

1. How

Answer 1

Location didn't work for me, here's what I did.

import 'dart:math' show cos, sqrt, asin;


double calculateDistanceBetweenTwoLatLongsInKm(
    double lat1, double lon1, double lat2, double lon2) {
  var p = 0.017453292519943295;
  var c = cos;
  var a = 0.5 -
      c((lat2 - lat1) * p) / 2 +
      c(lat1 * p) * c(lat2 * p) * (1 - c((lon2 - lon1) * p)) / 2;
  return 12742 * asin(sqrt(a));
}

Then just check if that distance is more (outside) or less (inside) than your radius in KM