Problem, when trying to differentate 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)

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

Answer 1

[Azzi Abdelmalek]

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

The result is

 b =

3*cos(3*x)

Answer 2

[Andy]

Please look at the picture attached! You will see what my problem is. Do you have any idea what's wrong?

Answer 3

[Andy]

Answer 4

[Walter Roberson]

I think maybe you had something in your workspace that was interfering. Did you try clearing all variables, or restarting MATLAB?

Answer 5

[Roberto]

try clearing the workspace

>> clear all;

Answer 6

[Andy]

Thank you! Clearing my workspace solved the problem.

[Azzi Abdelmalek]

Don't use clear all, just clear your variables