n=0:7;
m=0:7;
for i=1:4       
  x(i)=5-i;      
end 
for i=5:8       
  x(i)=i-4; 
end 
close all; subplot(2,1,1); stem(n,x); title('X3(t)的序列圖形');   
subplot(2,1,2); stem(m,abs(fft(x,8)));  title('X3(t)的序列8點的FFT');