Well, if you are going to calculate , then you can use the iterative formula , cause is approx as much lower than as is higher than it, or vice versa.
When dealing with there's not easy to find anything that is approximately as much lower than as is higher than it, or vice versa.
If , where , we can get an approximate value of , namely . I can show why:
Set , where .
But since we know that , we can put that into the equation:
, where
Since we now know that and that , we can dissolve as a linear combination of and .
And we get Hence we can use that for the next better value of .