Problem: Find the coefficient of x^7 in (1 + x + x^2 + x^3 + ...)^n(adsbygoogle = window.adsbygoogle || []).push({});

I thought this should be C(n + 6, 6) since you have to distribute 7 x's between the n series with repetition. But my book says it is C(n + 6, 7). (C for combination) What is the explanation?

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# Homework Help: Generating functions

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