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ta có $2^{2n+1}= C^{0}_{2n+1} + C^{1}_{2n+1}+......+C^{n}_{2n+1}+.....+C^{2n}_{2n+1} +c^{2n+1}_{2n+1}$
$C^{0}_{2n+1} = C^{2n+1}_{2n+1}$
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$C^{n}_{2n+1} = c^{n+1}_{2n+1}$
$\Rightarrow 2^{2n+1} = 2(C^{0}_{2n+1} +....+C^{n}_{2n+1})$
$\Rightarrow (C^{0}_{2n+1} +....+C^{n}_{2n+1}) = 2^{2n}$
$\Rightarrow C^{1}_{2n+1}+....C^{n}_{2n+1} = 2^{2n} -1=2^{20} -1$
$\Rightarrow n=10$
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