Differentation problem, when trying to diff sin(f(x)) with variable x

Hello!

I would like to try some basic functions in Matlab. I faced two problems, which seems unable to solve for me.

If I type:

a=sin(3*x)
b=diff(a,x)

Then I run it:

a=sin(3*x)(3*x)
b=3*D(sin(3*x))(3*x)

I think it should be:

a=sin(3*x)
b=3*cos(3*x)

I attach a picture, to see what my problem is.

Please tell me what is wrong with my little code! Thank you!

/matlabcentral/answers/uploaded_files/11907/pic1.jpg

[Walter Roberson]

duplicates http://www.mathworks.co.uk/matlabcentral/answers/127328-problem-when-trying-to-differentate-sin-f-x-with-variable-x