function f = Newtonback(x,y,x0)
syms t;

if(length(x) == length(y))
    n = length(x);
    c(1:n) = 0.0;
else
    disp('x和y的維數不相等！');
    return;
end

f = y(n);
y1 = 0;

xx =linspace(x(1),x(n),(x(2)-x(1)));
if(xx ~= x)
    disp('節點之間不是等距的！');
    return;
end

for(i=1:n-1)
    for(j=i+1:n)
        y1(j) = y(j)-y(j-1);
    end
    c(i) = y1(n); 
    l = t;
    for(k=1:i-1)
        l = l*(t+k);
    end;
    
    f = f + c(i)*l/factorial(i);
    simplify(f);
    y = y1;
    
    if(i==n-1)
        if(nargin == 3)
            f = subs(f,'t',(x0-x(n))/(x(2)-x(1)));
        else
            f = collect(f);
            f = vpa(f, 6);
        end
    end
end