X=backsub(A,B)

function X=backsub(A,B)

%Input  - A is an n x n upper-triangular nonsingular matrix
%	     - B is an n x 1 matrix
%Output - X is the solution to the linear system AX = B

%Find the dimension of B and initialize X

% NUMERICAL METHODS: MATLAB Programs
%(c) 1999 by John H. Mathews and Kurtis D. Fink
%To accompany the textbook:
%NUMERICAL METHODS Using MATLAB,
%by John H. Mathews and Kurtis D. Fink
%ISBN 0-13-270042-5, (c) 1999
%PRENTICE HALL, INC.
%Upper Saddle River, NJ 07458

 n=length(B);
 X=zeros(n,1);
 X(n)=B(n)/A(n,n);

for k=n-1:-1:1
 X(k)=(B(k)-A(k,k+1:n)*X(k+1:n))/A(k,k);
end