Help! Program involving 1st and 2nd Derivative

This is the Question:

On the interval [0, 2pi], plot the graph of f(x) = sin x + (1/3)sin(3x). In Matlab, find and plot all roots, relative extrema, and inflection points. Plot graphs of f' and f'' on separate axes.

My approach was to graph f, f', and f''. Then i tried to find the zeros of each graph and plot it on its respective curve. However I cannot get all the roots to plot.

This is my code http://postimg.org/image/oqbob6bpd/

This is my output http://postimg.org/image/hgrzljfo7/

Can you help me fix my problem or direct me to another approach that is easier?

Thanks

[sixwwwwww]

Dear Andrew, there is not one but multiple circles on each graph. You can zoom in then they will be visible separately as you can see in the following image I got by running you code:

Is it your desired solution?

[Andrew]

Oh I see, thank you. But, I am looking for a way to plot the graphs and put a red dot at each of the zeros on the curve.

[Walter Roberson]

Already answered below.

Answer 1

[Walter Roberson]

Change your

plot(fdprime)

to

plot(x, fdprime)

[Andrew]

Oh that did help me plot the points but I only get this http://postimg.org/image/epb0jqw8l/ not sure.

[Andrew]

Can you help me?

[Walter Roberson]

Change all of the plot(fdprime) to plot(x, fdprime)

[Andrew]

I still get the same error of "Vectors must be same length" for the 1st and 2nd derivative graph but it worked for the f graph

[Walter Roberson]

Consider changing your diff() to gradient()

[Andrew]

gradient() worked! Thank you very much Mr.Roberson