# Root finding and plotting

5 views (last 30 days)
WLL on 17 Jun 2021
Commented: Star Strider on 18 Jun 2021
How do I find the three roots of " f "
I got the plot of " f." And because the " f " and X asis has three intersections, it would have three roots. I would like to know how to find the three roots at the same time and plot the three roots on my original plot.
Below is my coding.
Thank you very much!!
clc
clear
format long
v=0.3;
E=209e+3;
G=E/(2*(1+v));
q=-1;
h=15;
D=(E*h^3)/(12*(1-v^2));
I=(h^3)/12;
a=600;b=2400;
n =3;
[T1, T2] = meshgrid(1:2:n);
mn = [T1(:), T2(:)]
syms x y
x_value=301;
y_value=0:1:2400;
len=length(mn);
amn=zeros(1,len);
for i=1:len
m=mn(i,1);
n=mn(i,2);
amn(i)=(16*q/(m*n*D*pi^6))*(1/((m/a)^2+(n/b)^2)^2);
end
wmn=sym(zeros(1,len));
wxx=sym(zeros(1,len));
wyy=sym(zeros(1,len));
for i=1:len
m=mn(i,1);
n=mn(i,2);
wmn(i)=amn(i).*((sin(m.*pi.*x./a)).*(sin(n.*pi.*y./b)));
wxx(i)=diff(wmn(i),x,2);
wyy(i)=diff(wmn(i),y,2);
end
combine_wxx=sum(wxx);
combine_wyy=sum(wyy);
My=-D*(combine_wyy+v*combine_wxx);
Differentiation_My=diff(My,y,1)
f=subs(Differentiation_My,x,301)
%fsolve(f,500)
Differentiation_My_value=double(subs(Differentiation_My,{x,y},{x_value,y_value}));
plot(Differentiation_My_value)
grid on

Star Strider on 17 Jun 2021
One way is to search for the indices nearest the zero-crossings, then interpolate (if necessary, to get more exact values) —
format long g
v=0.3;
E=209e+3;
G=E/(2*(1+v));
q=-1;
h=15;
D=(E*h^3)/(12*(1-v^2));
I=(h^3)/12;
a=600;b=2400;
n =3;
[T1, T2] = meshgrid(1:2:n);
mn = [T1(:), T2(:)]
mn = 4×2
1 1 1 3 3 1 3 3
syms x y
x_value=301;
y_value=0:1:2400;
len=length(mn);
amn=zeros(1,len);
for i=1:len
m=mn(i,1);
n=mn(i,2);
amn(i)=(16*q/(m*n*D*pi^6))*(1/((m/a)^2+(n/b)^2)^2);
end
wmn=sym(zeros(1,len));
wxx=sym(zeros(1,len));
wyy=sym(zeros(1,len));
for i=1:len
m=mn(i,1);
n=mn(i,2);
wmn(i)=amn(i).*((sin(m.*pi.*x./a)).*(sin(n.*pi.*y./b)));
wxx(i)=diff(wmn(i),x,2);
wyy(i)=diff(wmn(i),y,2);
end
combine_wxx=sum(wxx);
combine_wyy=sum(wyy);
My=-D*(combine_wyy+v*combine_wxx);
Differentiation_My=diff(My,y,1)
Differentiation_My = f=subs(Differentiation_My,x,301)
f = %fsolve(f,500)
Differentiation_My_value=double(subs(Differentiation_My,{x,y},{x_value,y_value}));
zxi = find(diff(sign(Differentiation_My_value))) % Zero-Crossing Indices
zxi = 1×4
583 1200 1201 1818
for k = 1:numel(zxi)
idxrng = max(zxi(k)-2,1):min(zxi(k)+2,numel(Differentiation_My_value)); % Index Range For Interpolation
zc(k) = interp1(Differentiation_My_value(idxrng),idxrng,0); % Interpolate
end
Zero_Crossings = zc
Zero_Crossings = 1×4
583.376183230044 1201 1201 1818.62381676996
plot(Differentiation_My_value)
hold on
plot(zc, zeros(size(zc)), 'rs')
hold off
grid on To plot it against an ‘x’ vector instead of the indices, the ‘zc’ calculation becomes:
% zc(k) = interp1(Differentiation_My_value(idxrng), x(idxrng), 0);
I commented it here because it would otherwise execute in the online Run feature, and throw an error.
.
Star Strider on 18 Jun 2021
I’ve definitely been there (although by now a few decades removed).
As always, my pleasure!

Sulaymon Eshkabilov on 17 Jun 2021
There are a couple of ways to find roots of this eqn:
(1) ginput() --> graphical method, e.g.:
[Roots, y] = ginput(3) % click on three crossing points
(2) fsolve()
WLL on 17 Jun 2021
Hi Sulaymon Eshkabilov,

R2021a

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