function X=trisys (A,D,C,B)%Input - A is the sub diagonal of the coefficient matrix%      - D is the main diagonal of the coefficient matrix%      - C is the super diagonal of the coefficient matrix%      - B is the constant vector of the linear system%Output - X is the solution vector%  NUMERICAL METHODS: Matlab Programs% (c) 2004 by John H. Mathews and Kurtis D. Fink%  Complementary Software to accompany the textbook:%  NUMERICAL METHODS: Using Matlab, Fourth Edition%  ISBN: 0-13-065248-2%  Prentice-Hall Pub. Inc.%  One Lake Street%  Upper Saddle River, NJ 07458N=length(B);for k=2:N   mult=A(k-1)/D(k-1);   D(k)=D(k)-mult*C(k-1);   B(k)=B(k)-mult*B(k-1);endX(N)=B(N)/D(N);for k= N-1:-1:1   X(k)=(B(k)-C(k)*X(k+1))/D(k);end