Cho x ˜ ( n ) = { 1 0 ≤ n ≤ 4 0 5 ≤ n ≤ 9 . Hãy xác định X ˜ ( k )

$\stackrel{}{X}\left(k\right)={\displaystyle \sum }_{n=0}\mathrm{Unsupported\; }e{}^{-j\frac{2\pi }{10}kn}=\frac{1-e{}^{-j\frac{2\pi }{10}k5}}{1-e{}^{-j\frac{2\pi }{10}k}}$

$\stackrel{}{X}\left(k\right)=\frac{\left(e{}^{\frac{j\beta \gamma }{2}}-e{}^{\frac{j\beta \gamma }{2}}\right)e{}^{\frac{j\beta \gamma }{2}}}{\left(e{}^{j,j\sigma }-e{}^{j,j\pi }\right)e{}^{-j,j\pi }}$

$\stackrel{}{X}\left(k\right)=\frac{\text{s}\text{i}\text{n}\frac{k\pi }{2}}{\text{s}\text{i}\text{n}\frac{k\pi }{10}}\cdot e{}^{-j\frac{2k\pi }{5}}$

Cả 03 phương án đều đúng