clear all
N =17; % number of approximations to calculate 
x0 =1;
exact = 9*x0^8;
h = 5*10.^(-(1:N));
df_dx = ((x0+h).^9 - x0^9)./h;
Er = abs(df_dx - exact);
loglog(h,Er,'-',h,Er,'*');
axis([1e-18 1 5e-8 1e2]);
xlabel('h')
ylabel('|error|');
set(gca,'Xtick',[1e-17 1e-13 1e-8 1],'Ytick',[1e-7 1e-3 1e1 1e2]); title('Error of forward differencing formula');
shg