error: fzero: zero point is not bracketed

2022 1 5
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更新时间：2022 1 5
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Hi. So this is what I wrote so far in octave:
syms x
f=4*x.^2+20*x+4
f = 
x=fzero(@(x) f, -5)
Error using fzero (line 308)
Initial function value must be finite and real.
but I keep getting the error in the title. What is wrong with what I wrote? Thanks in advance!

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回答（1 个）

Star Strider
Star Strider 2022-1-5
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Use fzero for numeric functions and solve for symbollic functions —
syms x
f=4*x.^2+20*x+4
f = 
x=vpa(solve(f==0))
x = 
format long
xd = double(x)
xd = 2×1
-4.791287847477920 -0.208712152522080
whos x xd
Name Size Bytes Class Attributes x 2x1 8 sym xd 2x1 16 double
.

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Andreea Oana
Andreea Oana 2022-1-5
How do I declare a numerical function?
Walter Roberson
Walter Roberson 2022-1-5
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syms x
f=4*x.^2+20*x+4
f = 
F = matlabFunction(f)
F = function_handle with value:
@(x)x.*2.0e+1+x.^2.*4.0+4.0
x = fzero(F, -5)
x = -4.7913
or
f = @(x) 4*x.^2 + 20*x + 4
f = function_handle with value:
@(x)4*x.^2+20*x+4
x = fzero(f, -5)
x = -4.7913
Star Strider
Star Strider 2022-1-5
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Ran in:
One approach —
syms x
f=4*x.^2+20*x+4
f = 
f_fcn = matlabFunction(f)
f_fcn = function_handle with value:
@(x)x.*2.0e+1+x.^2.*4.0+4.0
format long
x=fzero(f_fcn,-5)
x =
-4.791287847477920
To get the other root, use a different initial parameter estimate —
x=fzero(f_fcn,-1)
x =
-0.208712152522080
See the documentation on matlabFunction for details.
.

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