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diff --git a/3812/CH6/EX6.20/6_20.sce b/3812/CH6/EX6.20/6_20.sce
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+//example 6_20
+//find the fourier transform of the periodic signal x[n] = cos(Wo)n
+clear;
+close;
+clc;
+N = 5;
+n = 0:0.01:N;
+Wo = 2*%pi/N;
+xn =sin(Wo*n);
+for k =0:N-2
+  C(k+1,:) = exp(sqrt(-1)*Wo*n.*k);
+  a(k+1) = xn*C(k+1,:)'/length(n);
+  if(abs(a(k+1))<=0.1) 
+    a(k+1)=0;
+  end
+end
+a =a';
+a_conj =conj(a);
+ak = [a_conj($:-1:1),a(2:$)];
+Mag_ak = abs(ak);
+k = -(N-2):(N-2);
+plot2d3('gnn',k,Mag_ak,5)
+xtitle('abs(ak)','k','ak')