Fzero Initial value issues

hey guys getting an error when i try to run this ik the value is around 247 but im getting an error of

"Error using fzero

Initial function value must be finite and real."

%% Part A section II

dhdt = @(t,h) (((-100*r.^2)*sqrt(2*g*h))./(2*h*R-h.^2));

[t2,h2] = ode45(dhdt,[0:600],6.5);

plot(t2,h2);

axis([0 600 0 7])

fun = @(x) polyval(polyfit(t2,h2,1),x)-.5

minT = fzero(fun,245)

[John D'Errico]

Since you never tell us the value of r or g, we cannot even reproduce what you did. If you want help, it would be a good thing to make it easy to get help.

Answer 1

[Walter Roberson]

fun = @(x) polyval(polyfit(t2,h2,1),x)-.5

Why are you redoing the polyfit each time? You are not changing t2 or h2 or 1 inside fun so polyfit() is always going to return the same value, so you should pre-compute it.

You are fitting a straight line, getting out coefficients for something of the form a*x+b and you are wanting to find x such that a*x+b == 0.5 . But you can solve that directly without fzero, as x = (0.5-b)./a .

p = polyfit(t2, h2, 1);
x = (0.5 - p(2))./p(1);