证明: $\int_0^1\left(1+\sin \frac{\pi}{2} x\right)^n \mathrm{~d} x>\frac{2^{n+1}-1}{n+1} \quad(n=1,2, \cdots)$;
(2) 求极限 $\lim _{n \rightarrow \infty}\left[\int_0^1\left(1+\sin \frac{\pi}{2} x\right)^n \mathrm{~d} x\right]^{\frac{1}{n}}$ 。
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