Solving equation, calculus derivatives

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shelly
shelly 2013-2-17
编辑： Pascal André 2015-10-2
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.

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回答（1 个）

Walter Roberson
Walter Roberson 2013-2-17
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)
  1 个评论
Pascal André
Pascal André 2015-10-2
编辑：Pascal André 2015-10-2
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.

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