Chứng minh các đẳng thức sau:
$2) 2^nC_{n}^{0}+2^{n-1}.7^1C_{n}^{1}+...+2.7^{n-1}C_{n}^{n-1}+7^{n}C_{n}^{n}=9^n$
$2^nC_{n}^{0}+2^{n-1}.7^1C_{n}^{1}+...+2.7^{n-1}C_{n}^{n-1}+7^{n}C_{n}^{n} = \sum_{k=0}^n C_n^k.2^{n-k}.7^k = (2+7)^n = 9^n$
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