n=0:7;
m=0:15;
for i=1:4  
  x(i)=i;  
end 
for i=5:8      
  x(i)=9-i; 
end 
close all; subplot(2,1,1); stem(n,x);title('X2(t)的序列圖形');  
subplot(2,1,2); stem(m,abs(fft(x,16)));title('X2(t)的序列16點的FFT');