Chapter 4, Problem 2PE

(Geometry: great circle distance) The great circle distance is the distance between two points on the surface of a sphere. Let $\left(x\text{l},y\text{l}\right)\text{and}\left(x2,y2\right)$ be the geographical latitude and longitude of two points. The great circle distance between the two points can be computed using the following formula:

$d=radius\times \arccos (\sin (x_1)\times \sin (x_2)+\cos (x_1)\times \cos (x_2)\times \cos (y_1-y_2))$

Write a program that prompts the user to enter the latitude and longitude of two points on the earth in degrees and displays its great circle distance. The average earth radius is 6,378.1 km. The latitude and longitude degrees in the formula are for north and west. Use negative to indicate south and east degrees.