Casio fx-83ES-the one with a purple button at the top - fault or is it me?

I have a lovely Casio calculator, which does lots of wonderful things for me, BUT, it has also driven me to distraction, because it keeps miscalculating one particular , um, what do you call it, (sum?)

I've tried doing the same thing on my other (older) Casio calculator, and it comes out OK on that. The particular problem involves doing the following:

20 squared add 20 squared, then finding the square root of this answer, it keeps coming out as 5 point something instead of 28 point something

new note:

Actually, I've just found a way round this , which is to completely bypass the surd answer the calculator gives, by pressing "Shift, equals", from the start, in order to get a decimal answer right from the start.

The trouble with complicated calculators like this is, while such complexity can be useful, it can also be complex to operate, and thus easy to go wrong.]