Yet another method for Pi - this time from the outside!

This week's challenge is about doing exactly what we did last week - but from the outside of the circumference. It is not as easy as it looks at first sight - and I am not alone in thinking that the inner polygon approach is easier than the outer approach. Let's first lay down the equations after seeing the enclosed drawing. From there, you need to draw the iterative formula, and then program it on the HP calculator of your choice.

(2a)    

(2b)    

There you are! Let's try!