1) ??

2) squareroot ==> sqrt(number);

3) cuberoot ==> cbrt(number);

4) quadroot ==> sqrt(sqrt(number));

5) pentaroot ==> pow(number, 0.2);

6) sixtroot ==> cbrt(sqrt(number));

...

in general

`double nthroot(double number, double n)`

{

if (n == 0) return NAN;

if (number > 0) return pow(number, 1.0 / n);

if (number == 0) return 0;

if (number < 0 && int(n) == n && (int(n) & 1)) return -pow(-number, 1.0 / n);

return NAN;

}