Determining where y = 0

Hi there, I have a function f(x) and need to determine where f(x) is increasing and decreasing

So I determined the derivitive of f(x) as Df.

I need to get the values of x where y=0 of Df to 4 decimal places.

I am asking if someone could help me.

1. To get the points where Df(x) = 0
2. To get these values rounded to 4 decimal spaces

This is the graph of Df

Code i used to get here:

syms f(x)
f(x) = x.*(cos(x.^2)) - exp(sqrt(x)) + x.^3 - 4*(x.^2);
Df = diff(f,x);
fplot(Df, [0 4], 'b')

grid on

xlim([0.00 4.00])
ylim([0.0 27.3])

Answer 1

[KSSV]

Try this:

syms f(x)
f(x) = x.*(cos(x.^2)) - exp(sqrt(x)) + x.^3 - 4*(x.^2);
Df = diff(f,x);
y = subs(Df,x,0:0.001:4) ;  % this should be an array 
idx = abs(y)<=10^-3 ; 
iwant = y(idx) ;

fplot(Df, [0 4], 'b')

grid on

xlim([0.00 4.00])
ylim([0.0 27.3])

[Duncan Weakley]

[Duncan Weakley]

Ive herd of a function called chebfun? trying it now, but no success so far :(

If you have any more ideas, i would greatly appreciate it

[KSSV]

Ohk..there x in the denominator....

start from 0.01 or some number different from zero.

[Duncan Weakley]

i did this, but the live editor has been running for 15 minutes without output, think its to much to handle...

i resolved the issue by using chebfun:

[Duncan Weakley]

although i still get that weird warning... i only needed the roots in the interval [0 4], so i got what i needed thank you!