function R=xiaoqu(R,k)
%多元逐步回歸分析
%對R作消去變換

G=1/R(k,k);
m=length(R);
for i=1:m
    for j=1:m
        if i~=k&j~=k
            R(i,j)=R(i,j)-R(i,k)*R(k,j)*G;
        end
    end
end
for i=1:m
    R(k,i)=R(k,i)*G;
    R(i,k)=-R(i,k)*G;
end
R(k,k)=G;