function    Ja(A,b)
%Jacobi迭代法求方程組
D=diag(diag(A))
n=size(A)
if det(D)==0
    error('it is not situed for Jacobi method')
end
I=eye(n)
B=I-inv(D)*A
g=inv(D)*b
Nj=0;
x0=zeros(n,1);
while(Nj<100)
    xj=B*x0+g;
    if (norm(xj-x0)<0.00001)break
    else x0=xj
    Nj=Nj+1    
    end
end