error: fzero: zero point is not bracketed

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!

Answer 1

[Star Strider]

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              

.

[Andreea Oana]

How do I declare a numerical function?

[Walter Roberson]

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]

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.

.