Calculate distance of two geo points in km c#

Question

I`d like calculate the distance of two geo points. the points are given in longitude and latitude.

the coordinates are:

point 1: 36.578581, -118.291994

Answer 1

I used the formula from Wikipedia and put it in a lambda function:

Func CalcDistance = (lat1, lon1, lat2, lon2) =>
        {
            Func Radians = (angle) =>
            {
                return angle * (180.0 / Math.PI);
            };

            const double radius = 6371;

            double delataSigma = Math.Acos(Math.Sin(Radians(lat1)) * Math.Sin(Radians(lat2)) +
                Math.Cos(Radians(lat1)) * Math.Cos(Radians(lat2)) * Math.Cos(Math.Abs(Radians(lon2) - Radians(lon1))));

            double distance = radius * delataSigma;

            return distance;
        };