Derivative of a function with value of x

Question

CaiX 2020-11-10

回答： John D'Errico 2020-11-11

Hi, I've been trying to code for the derivative of ln(x), where x is a value to be set by the user. Here's what I've done so far..

x = input('Please enter value of x: \n');
f = log(x)
y = diff(f)
disp(y) 

I'm not quite sure if im on the right track... should I use

syms x

or

diff(f,x)

I would greatly appreciate any help/explanation. Thank you and stay safe!

Answer 1

John D'Errico 2020-11-11

You want to find the derivative of a function. First, create the function in a symbolic form.

syms x
f = log(x);

So f is a variable, containing the "function" we want. First, differentiate it.

df = diff(f,x);

So df is a variable. It contains the derivative, but it is not truly a function.

Now, choose a point to evaluate the function at.

x0 = input('Please enter value of x: \n');

And substitute into df. We can use subs for that.

subs(df,x,x0)

Or, if you prefer, we can convert df into a function.

df_fun = matlabFunction(df,x);
df_fun(x0)