Math Help - When is it true (x+y)^n=x^n + y^n ??

1. When is it true (x+y)^n=x^n + y^n ??

When is it true (x+y)^n=x^n + y^n ?? Any thoughts?

2. It's called Child's Binomial Theorem. If $n = 1$, it's true. If $n \ge 2$, not so much. For all [LaTeX ERROR: Convert failed] , though, $\displaystyle (x+y)^n = \sum_{k=0}^n {n \choose k}x^{n-k}y^k$ (call it Grown-up's Binomial Theorem).

3. That's what I thought, only n=1. Thanks

4. Also of course when x=0 or y=0.