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diff --git a/40/CH4/EX4.20/Exa_4_20.sce b/40/CH4/EX4.20/Exa_4_20.sce
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+//z transform of switched periodic signals
+z=%z;
+//sol. for 4.20a
+x1=[0 1 2];
+n=0:2;
+N=3;
+x1z=x1*(1/z)^n'
+xz=x1z/(1-z^-N)
+//sol.for 4.20b
+x1=[0 1 0 -1];
+n=0:3;
+N=4;
+x1z=x1*(1/z)^n'
+xz=x1z/(1-z^-N)
+//sol.for 4.20c
+xz=(2+z^-1)/(1-z^-3);
+x1z=numer(xz)
+//thus first period of xn is [2 1 0]
+//sol.for 4.20d
+xz=(z^-1-z^-4)/(1-z^-6);
+x1z=numer(xz)
+//thus first period of xn is [0 1 0 0 -1 0]
+\ No newline at end of file