Question

How to find root 21
Thanks

Answer 1

√(21) = √(21), it's an irrational number so you can only hope to get a estimate, calculator or not. 

4² < x² < 5² 
16 < x² < 25 
4 < x < 5 

So √(21) is between 4 and 5. (-4 and -5 work too, but in most cases you are only concerned about the principal root). 

The value seems to be leaning towards 5, so it's greater than 4.5, but not too great. If you do put in the calculator you see its ≈ 4.58.