clear all; home;
%4a
n = (2:99)';

c = [cos(n); 0; 0];
d = [1; -(ones(size(n),1)); 1];
a = [0;0; sin(n)];

b = [ 1; zeros(size(n),1); 1];

A = spdiags([c d a], -1:1, 100,100);
x = A\b;

%4b
co = cos(n);
si = sin(n);
plot(2:99,log10(abs((co.*x(n-1)) + x(n))./abs((co.*x(n+1))-x(n))),'r-');
hold on;
plot(2:99,log10(abs((co.*x(n-1)) + (x(n)+si.*x(n+1))./abs((co.*x(n+1))-(x(n)+si.*x(n+1))))),'b-');