The *antiderivative* of x^n is any function x^(n+1)/(n+1)+c, because the derivative of x^(n+1)/(n+1)+c is x^n.
The reason the derivative of x^(n+1)/(n+1)+c is x^n is [binomial theorem](https://en.wikipedia.org/wiki/Power_rule#Proof_by_binomial_theorem_(natural_numbers)).
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