Why is the Excel calculation error high?

Question

excel

First option
1. I take the root of the sum of squares of a pair of values
​​2. I take the root of the sum of squares of the second pair of values
​​3. I sum the values ​​and get the result
The second option
1. I sum the first and second values ​​in pairs
2. I take the root of the sum of squares, I get a different result
Questions A :
1.How to identify the method with the highest accuracy? What method is used when calculating roots in excel? And how can you improve the accuracy in excel?
2. Perhaps I have already "went" and forgot how to count

Answer 1

The method with the highest accuracy can be revealed with a pencil and paper.
However, we do not know what you need to calculate.
And the options are different, which is already there.