Question

i want to know that how to learn square value and cube value and how find large digit of underroot

Answer 1

don't learn square or cube of some number means the two times or three times of that number respectively
like     2² = 2 × 2 = 4       2³ = 2 × 2 × 2 = 8
 3² = 3 × 3 = 9             3³ = 3 × 3 × 3 = 27
to do square root bigger values just prime factorize it
like for number 17424 
after prime factorization we get 17454 = 11 × 11 × 2 × 2 × 2 × 2 × 3 × 3
so square root =√(11 × 11 × 2 × 2 × 2 × 2 × 3 × 3)
very very two same numbers under root you will get 1 number outside root
that is     = √(11 × 11 × 2 × 2 × 2 × 2 × 3 × 3)
                  = 11× 2 × 2 × 3
                    =132
some time a case like this can happen like for square root of this number
98
so √98
 =√( 2 × 7 × 7 )
= 7√2 
now you require value of √2 which you can get by root division 
√2 = 1.414
so 7√2 = 7 × 1.414 = 9.898

Answer 2

Just split the numbers under root and solve it and just leave an irrational number as it is