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HINT: $$\sum_{n=5}^9e^{-j\pi kn/5}=\sum_{n=0}^4e^{-j\pi k(n+5)/5}=\sum_{n=0}^4e^{-j\pi kn/5}e^{-j\pi k}=(-1)^k\sum_{n=0}^4e^{-j\pi kn/5}$$ I'm sure you can take it from here.


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