Are you familiar with the Haversine formula?.
Let lat1=u, long1=v, lat2=x, long2=y
Multiply this by the radius of the Earth and it should get you close.
Use the points:
C ( 40 N , 70 W ) D ( 40 N , 150 W )E( 70 N , 150 W ) F ( 70 N , 70 W )
to find the distance from C to E along a great circle.
( Hint: You will need to find the lengths of the Trapezium CDEF)
Any Contribution and Help will be Appreciated.