Solving equation, calculus derivatives

I really am stuck on this question, any help appreciated,

For the function f(x) = (exp(-2*x)-2.7*x^2)/(cosh(3*x)

find the coordinates of the point with x>0 at which f has a zero derivative.

x =?
y=?

I know that I have to differentiate the function, and than set the derivative to zero, however I canot seem to set f'(x)=0 on matlab, this is what I have done so far.

syms x 
f=inline((exp(-2*x)-2.7*x^2)/(cosh(3*x)))
df=diff(f(X),x)

and than i try and do fzero(df,1) on matlab, to set the derivative to zero, and than find he value of x, which I will than put back into the derivative to get the value of y. However i am really stuck from this point onwards.

Any help appreciated.

Answer 1

[Walter Roberson]

There is no reason to inline()

syms x
f = (exp(-2*x)-2.7*x^2)/(cosh(3*x));
df = diff(f, x);
solve(df, x)

[Pascal André]

Pay attention to syms and double:

Example:

syms t

x=100*t

dx=diff(x,t)

y=100*t-9.81*t^2

dy=diff(y,t)

timpact=max(double((solve(y,t))))

xmax=subs(x,t,timpact)

thmax=double(solve(dy,t))

ymax=subs(y,t,thmax)

I hope to have contributed a little cause I am a beginner.