The Binomial Expansion (1 + a)n is not always true.
It is valid for all positive integer values of n.
But if n is negative or a rational value then it is only valid for 1 < a < -1

In the next tutorial you are shown how we can work out the range of values of taken by x in a Binomial expansion that has rational powers.

Binomial Expansion | Validity for Rational Powers | ExamSolutions - youtube Video