Bài toán: Xét khai triển sau :
$$(1+x+x^2+...+x^{m})^{n+1}=a_0+a_1x+...+a_{m}x^{m}+...+a_{m(n+1)}x^{m(n+1)}(m,n \in \mathbb{N})$$
Chứng minh rằng :
$$\sum_{k=0}^{m}a_{k}=\binom{n+1+m}{n+1}$$
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