Tìm X ( k ) trong trường hợp x ( n ) = r e c t 4 ( n )

$X\left(k\right)=\{\begin{array}{c}4\\ \\ 0\\ \text{k}\text{c}\text{ò}\text{n}\text{l}\text{a}\text{i}\end{array}$

$X\left(k\right)=\{\begin{array}{c}1\\ 0\le k\le 4\\ 0\\ \text{k}\text{c}\text{ò}\text{n}\text{l}\text{a}\text{i}\end{array}$

$X\left(k\right)=e{}^{-j\frac{2k\pi }{4}}$

$X\left(k\right)=\{\begin{array}{c}4\\ k=0\\ 0\\ k\ne 0\end{array}$