function r=newtondf1(x,y)
    nx=length(x);
    for i=1:(nx-1)
        t=x(i)+0.1;
        if(i~=1)
            for k=1:3
                x1(k)=x(i+k-2);
                y1(k)=y(i+k-2);
            end
            s=lagr(x1,y1,t);
        else 
            for k=1:3
                x1(k)=x(k);
                y1(k)=y(k);
            end
            s=lagr(x1,y1,t);
        end
        r(i)=(s-y(i))/(t-x(i));         
    end