function  [a,d]=chashang(f,x,y,x0)
%差商
m=length(x);
n=length(y);
a=zeros(n,n);
if n~=m
error('the lengthof x and y is unequal!');
return;
end
for i=1:n
a(i,1)=y(i,1);
end
for j=2:1:n
for i=j:1:n
a(i,j)=(a(i,j-1)-a(i-1,j-1))/(x(i,1)-x(i-j+1,1));
end
end
d=a(1,1);
for i=2:1:f+1
c=1;
for j=1:1:i-1
c=c*(x0-x(j));
end
end