Find zero of a polynomial using fzero

I am trying to find the zeros of deflecPrime using fzero (not roots).

deflecPrime=(w/(48*E*I)).*[8 -15*L 6*L^2 0];

%zero of deflection derivative

fun=@deflecPrime;

x0=L/2;

maxDeflec=fzero(fun,x0)

I keep getting this error:

FZERO cannot continue because user-supplied function_handle ==> deflecPrime failed with the

error below.

Undefined function 'deflecPrime' for input arguments of type 'double'.

[Walter Roberson]

By the way, with respect to your recent question that you deleted:

[X,Y]=meshgrid(x,y);

X and Y will both be 2D arrays.

f=2.25*X.*Y+1.75*Y-1.5*X.^2-2*Y.^2;

f is built from 2D arrays, so f will be a 2D array.

fun = @(z)-2.25*X.*Y-1.75*Y+1.5*X.^2+2*Y.^2;

You accept an input but ignore it and always return the same thing that was calculated and stored into f.

You could change the @(z) to @(f) but that would not make any difference.

z = fminsearch(fun,x0)

the function you pass to fminsearch must return a scalar, not an array.

I suspect what you want is

fun = @(XY)-2.25*XY(1).*XY(2)-1.75*XY(2)+1.5*XY(1).^2+2*XY(2).^2;

Note, by the way, that you can calculate the optimum using calculus instead of fminsearch. When f is of the form

A*X.*Y + B*Y - C*X.^2 - D*Y.^2

then

x = -(A*B)/(A^2 - 4*C*D)
y = -(2*B*C)/(A^2 - 4*C*D)

is the critical location. (Caution: you really should check whether it is a minima or maxima)

Answer 1

[Ameer Hamza]

You need to define deflecPrime as a function handle

deflecPrime = @(L) sum((w/(48*E*I)).*[8 -15*L 6*L^2 0]); % sum all the terms too

and then call fzero like this

fun = deflecPrime;
x0=L/2;
maxDeflec=fzero(fun,x0)

However, fzero will not give al the roots. For that, use roots(): https://www.mathworks.com/help/matlab/ref/roots.html.

deflecPrime = (w/(48*E*I)).*[6 -15 8] % [cofficients of x^2, x, 1]
roots(deflecPrime)

[Rachel A. Eaglin]

I created this code:

dP= @(L) (w/(48*E*I)).*[8 -15*L 6*L^2 0];

fun=dP;

x0=L/2;

maxDeflec=fzero(fun,x0)

and now this error:

Operands to the || and && operators must be convertible to logical scalar values.

Error in fzero (line 327)

elseif ~isfinite(fx) || ~isreal(fx)

Error in HW3Script (line 30)

maxDeflec=fzero(fun,x0)

[Rachel A. Eaglin]

Got it! Thank you!

[Ameer Hamza]

I am glad to be of help!