Kth root in sorting and searching

#1

Question : You are given two integers n and k. Find the greatest integer x, such that, x^k <= n.

I am using this formula to calculate x
x = 10 ^ (log10(n) / k)
x is int so decimal part automatically is taken care of.
Test Case 2 is not working Why?

#2

Take an example
x^3<=15
3log10(x)<=log10(15)
x<=10^(log10(15)/3)
x<=10^(1.17/3)
x<=10^(0.39)

So how are you computing 10^(0.39) is actually my concern.
Please add the code and the problem link.