Hello, this is my first post in this site, so here goes: Im working on a Matlab program. I have to calculate the second derivative of a given equation. The problem is that plotting the derivative is necessary, and that's where I'm stuck. Here's what I've got so far: % Evaluar f(x) y f'(x) usando diferencias hacia atras. % x = -10:0.1:10; f = x.^3 - 5*x.^2 + 2*x + 8; df = diff(f); dx = diff(x); df_dx = df./dx; xd = x(2:length(x)); d2f = diff(f,2); d2x = diff(f,2); d2f_d2x = d2f./d2x; % % Graficar lo anterior. % subplot(2,1,1),... plot(x,f,xd,df_dx,xd,d2f_d2x),... title('Derivada de un polinomio de quinto grado'),... xlabel('x'),grid,... ylabel('f(x)'),... axis([-4,5,-500,1500]); On the internet I found out that diff(f,2) would calculate the second diff, I guess thats true. Ignore the spanish text :P. When I try to run the program, the following appears: ??? Error using ==> plot Vectors must be the same lengths. Error in ==> C:\MATLAB6p5\work\segunda_diferencial.m On line 16 ==> plot(x,f,xd,df_dx,xd,d2f_d2x),... Hope you can help. Thanks!