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Determining where y = 0

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Duncan Weakley
Duncan Weakley 2020-12-3
评论： Duncan Weakley 2020-12-3
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])

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采纳的回答

KSSV
KSSV 2020-12-3
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])
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Duncan Weakley
Duncan Weakley 2020-12-3
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
Duncan Weakley 2020-12-3
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!

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