The best approximation is, of course, the exact value. A reasonable approximation is 106.
What approximation is "best" depends on the required level of accuracy.
Z Series are the best
It is 60
There is no "best" approximation. The square root is irrational and so it is a non-terminating, non-recurring decimal so adding one more digit will always make it a better approximation.
20.1094005
11.5326
20.1
√40108 = 200.269817996
3.16227766 which is about 3.2
It can be used in function approximation, especially in physics and numerical analysis and system & signals. Actually, the essence is that the basis of series is orthorgonal.
Depends on the accuracy required in the result.