function a=ployfit(x,y,w)  %最小二乘擬合
m=length(w);
G=zeros(m);
D=zeros(m,1);
for i=1:m
    for j=1:m
        G(i,j)=sum(w.*(x.^(i+j-2)));
    end
    D(i,1)=sum(w.*y.*(x.^(i-1)));
end
G=G(1:2,1:2)
D=D(1:2)
  a=G^(-1) *D