FZero NaN help!

Iv got this question: Consider the curve defined by y = 40*cos(0.1x + 0.2). The distance d(x) from the origin (0, 0) to a point (x, y) on the curve is

d(x) = sqrt(x^2 + (40*cos(0.1x + 0.2))^2)

(a) Write a MATLAB program and function M-file to plot d(x) for −100 ≤ x ≤ 100. Also use the function M-file and the fminbnd command to find (and display) the point (xmin,ymin) on the curve that is closest to the origin and display the minimum distance (i.e., the distance from the origin to the point (xmin,ymin)).

(b) Write a MATLAB program that uses the fzero command to find the point(s) on the curve at a distance of 60 from the origin.

i have a function file:

function D = q4(x)

D = sqrt(x.^2 + ((40*cos((0.1*x) + 0.2)).^2));

part a is this:

clear

format short

x = (-100:0.0001:100);

D=q4(x); %Sending x values to function

plot(x,D)

xlabel('d(x)')

ylabel('Y')

grid

pause

x1=input('Enter left hand endpoint of subinterval: ');

x2=input('Enter right hand endpoint of subinterval: ');

xmin=fminbnd('q4', x1,x2);%Finding xmin

ymin=q4(xmin);%finding ymin

hold on;

plot(xmin,ymin,'rO')%showing point on plot

dis=xmin;

%display functions

disp('xmin');

disp(xmin);

disp('ymin');

disp(ymin);

disp('Distance from the origin: ');

disp(dis);

the only issue i have is part B where using fzero, as the function doesnt cut the x axis i didnt think you could run it. My lecturer said it needs to be when d(x)=60 but im unsure of how to go about doing that. Also d(x)-60 =0 is also taken into account.

[Aaron Brown]

Hi, did you end up getting this sorted. I have run into a similar issue.

Answer 1

[Star Strider]

Solving for ‘d(x)-60’ is the correct approach.

Once you have written your anonymous function to define ‘d(x)’, you need to write a second one (it can be a direct argument to the fzero function) to create the 60 offset:

x0 = initial guess;
x60 = fzero(@(x) d(x)-60, x0)

[Bryce Standkey]

clear

format short

x = (-100:1:100);

D=q4(x); %Sending x values to function

plot(x,D)

xlabel('d(x)')

ylabel('Y')

grid

x0 = 55;

x60 = fzero(@(x) D-60, x0);

returns error: >> q4b Error using fzero (line 167) FUN must be a function, a valid string expression, or an inline function object.

Error in q4b (line 13) x60 = fzero(D-60, x0);

[Star Strider]

Anonymous Functions