If you use rational fraction approximations for pi, the Tsu Chi form, 355/113, is far more accurate than 22/7. There are other approximations that are even more accurate as this extract from my notes shows...
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APPROXIMATING PI = 3.1415926535898
Biblical approximation: 3 (4.5%)
"And he [Hiram] made a molten sea, ten cubits from the one rim to the other it was round all about, and...a line of thirty cubits did compass it round about....And it was an hand breadth thick...." — First Kings, chapter 7, verses 23 and 26
22 / 7 = 3.142857142857 (0.04 %)
22 / 7.0028 = 3.141600502656 (0.00025 %)
Tsu Chung Chi approximation: 355 / 113 = 3.141592920354 (0.0000085 %)
Euler: 103993 / 33102 = 3.141592653012 (-0.000000018 %)
Ramanujan's improvement to Chi: (355 / 113) * (1 - 0.0003 / 3533) = 3.14159265359 (below calculator range)
Ramanujan's first term in fast series for pi: 9801 / [1103 * sqrt(8)] = 3.141592730013 (0.0000024 %)
Ramanujan: (63 / 25) * [(17 + 15 * sqrt(5)) / (7 + 15 * sqrt(5))] = 3.141592653806 (0.000000007 %)
Fourth root: (2143 / 22)^(1/4) = 3.14159265258 (-0.000000032 %)
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Four bangers, despite having a square root key, usually lack the far more useful pi key, so knowing some easy approximations is a good thing.
Newton's technique of iterating to find other roots can easily be generalized. For the nth root of N, the equation is:
x2 = [(n - 1)*x1 + N/x1^(n-1)]/n
which, for n = 2, reduces to the equation shown in my previous post.
The convergence is at least quadratic in a neighborhood of the zero, which intuitively means that the number of correct digits roughly doubles in every step. For faster convergence, one can use Halley's method which exhibits cubic convergence.

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