例#:已知N的x(n)序列的DFT如图所示,求下图x1,x2, 3,,x4的FT x(nt FT X(k) 0 XI=x(n 9+a.X(k) 0 x2 k . ft mm Y(k) NT=T 0 k例#：已知N 的x(n)序列的 DFT如图所示，求下图x1，x2， x3, , x4的FT 0 N 0 N n k x(n) X (k) x1 x (n) = p n 0 N x2 x (t) = p NTs = T 0 2 N N 2 N − X (k) 0 2 N 2 N − ( ) 2 X k N  k k  1  N 2 DFT FT FT