Some property of ideals

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We have that $I$ is an ideal in ring $R$. How prove: if $x^a \in I$ and $y^b \in I$ show that $(x+y)^{(a+b)} \in I$? I don't have any idea. I know what is it ideal

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Hint: using the binomial expansion: $(x+y)^{a+b}=\sum_{k=0}^{a+b}{a+b\choose k}x^ky^{a+b-k}$. Can you conclude from here?