A = [ 1 2 0 0 0
        3 5 3 0 0
        0 2 3 3 0
        0 0 1 1 3
        0 0 0 1 1];
% вектор-столбец правой части
B = [2
     4
     3
     3
     4];
 
[n,n]=size(A);
 
P = zeros(1,n-1);
Q = zeros(1,n);
zn = A(1);
i = 1;
Q(1) = B(1)/zn;
 
while(i<n)
      if (i==1)
          P(i) = -A(i,i+1)/zn;
      else
        zn = A(i,i)+A(i,i-1)*P(i-1);
        P(i) = -A(i,i+1)/zn;
        Q(i)= (B(i)-A(i,i-1)*Q(i-1))/zn;
      end
        i=i+1;
end
zn =  A(n,n)+A(n,n-1)*P(n-1);
Q(n)=(B(n)-A(n,n-1)*Q(n-1))/zn;
X(n)=Q(n);
for i=n-1:-1:1
X(i)=P(i)*X(i+1)+Q(i);
end 
 
disp(X)

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