A power series always converges absolutely inside its radius of convergence and the standard method of finding that is to apply the "ratio test" or the "root test" to the absolute value of the series. Normally, the ratio test is easier to apply but those powers of n make this ideal for the root test!
so the nth root is . This will converge absolutely if and only if the limit of that, as n goes to infinity, is less than 1:
Now it should be clear what the radius of convergence and the interval of convergence is.