a = 3, b = 5

n    an                   bn
1    4.0                  3.872983346207417
2    3.9364916731037085   3.935979342530861
3    3.936235507817285    3.9362354994818265
4    3.9362355036495558   3.9362355036495558
5    3.9362355036495558   3.9362355036495558
6    3.9362355036495558   3.9362355036495558
7    3.9362355036495558   3.9362355036495558
8    3.9362355036495558   3.9362355036495558
9    3.9362355036495558   3.9362355036495558