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function X=trisys(A,D,C,B); 
N=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); 
end 
X(N)=B(N)/D(N); 
for k=N-1:-1:1 
    X(k)=(B(k)-C(k)*X(k+1))/D(k) 
end