# How do I plot and return the values of multiple intersections between a function and zero?

2 views (last 30 days)
Gerry Oscar on 20 Oct 2021
Commented: Gerry Oscar on 21 Oct 2021
Hello,
I need to find the intersection between the det_a function and zero, return the values, and plot the intersections. Here's what I did so far
x = linspace(0,18,1000);
det_a = sin(x).*cosh(x) - cos(x).*sinh(x);
zr = zeros(size(x));
figure
plot(x,det_a,x,zr,'--')
grid on
ylim([-10 10])
xlabel('\betal')
The plot can be seen here: I haven't found any easy method of doing this, I've tried using the find and intersect functions but they only return one value. Any help would be appreciated, thanks in advance!

Alan Stevens on 20 Oct 2021
Here's one possibility (though you get repeated results for the roots!):
det_a = @(x) sin(x).*cosh(x) - cos(x).*sinh(x);
x=0:18;
zr = zeros(1,numel(x));
for i = 1:numel(x)
zr(i) = fzero(det_a, x(i));
end
disp(zr)
0 -0.0000 0.0000 3.9266 3.9266 3.9266 7.0686 7.0686 7.0686 10.2102 10.2102 10.2102 13.3518 13.3518 13.3518 13.3518 16.4934 16.4934 16.4934
figure
xx = linspace(0,18,100);
plot(xx,det_a(xx),zr,0,'o')
grid on
ylim([-10 10])
xlabel('\betal') Gerry Oscar on 21 Oct 2021
Riight, thanks!

KSSV on 20 Oct 2021

R2019a

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!