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

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Gerry Oscar
Gerry Oscar 2021-10-20
评论： Gerry Oscar 2021-10-21
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!

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采纳的回答

Alan Stevens
Alan Stevens 2021-10-20
Ran in:
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')
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Alan Stevens
Alan Stevens 2021-10-20
Ran in:
fzero finds the value of x that makes the function det_a equal zero. So if it makes det_a+2 equal zero, then det_a must be -2.
Something like this:
det_a = @(x) sin(x).*cosh(x) - cos(x).*sinh(x);
det_b = @(x) det_a(x) + 2;
x=0:18;
zr = zeros(1,numel(x));
for i = 1:numel(x)
zr(i) = fzero(det_b, x(i));
end
disp(zr)
-1.4526 -1.4526 3.9795 3.9795 3.9795 3.9795 7.0662 7.0662 7.0662 10.2103 10.2103 10.2103 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,-2,'o')
grid on
ylim([-10 10])
xlabel('\betal')

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KSSV
KSSV 2021-10-20

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