function J=build_jacobian(xs,ts)
    m = length(ts);
    J = zeros(m,2);
    for i=1:m
        J(i,1) = -ts(i)/(xs(2) + ts(i));
        J(i,2) = xs(1)*ts(i)/(xs(2) + ts(i))^2;  
    end
end
