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Symbol calculation and numerical values

Asked by Pouyan Msgn on 12 Jul 2019
Latest activity Commented on by John D'Errico
on 14 Jul 2019
Hi!
I have my code here :
clc
clear all
syms k w m e s
eqn=k==[w*sqrt(e*m/2)*sqrt(sqrt(1+(s/(w*e))^2)-1)]
k=1; e=0.0006; m=2000; w=2*pi*10^6;
sol=solve(eqn,s)
It will solve s for me, but if I want now a value for s, how should I do that? I do not just want to have S as a symbol, but when I have values for other parameters S must also get a numeric value. How can I do this in Matlab?

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1 Answer

Answer by John D'Errico
on 12 Jul 2019
 Accepted Answer

You are doing things in the wrong order.
k,e,m,w are not unknowns. They are knowns. So why have you set them up as symbolic?
Next, subtract k from the right hand side. Don't set the two equal. This lets you plot the thing. ALWAYS PLOT EVERYTHING! Do that before you just throw it into a solver.
syms s
k=1; e=0.0006; m=2000; w=2*pi*10^6;
eqn = (w*sqrt(e*m/2)*sqrt(sqrt(1+(s/(w*e))^2)-1)) - k;
fplot(eqn,[-.1,.1])
yline(0);
grid on
I've plotted it over a fairly small domain. It does cross the line at y==0.
untitled.jpg
ssol = solve(eqn,s)
ssol =
-(1288490188800*pi*3^(1/2)*18206206649565666255441706068998^(1/2))/27309309974347922922410255680009
(1288490188800*pi*3^(1/2)*18206206649565666255441706068998^(1/2))/27309309974347922922410255680009
vpa(ssol)
ans =
-0.0010954451150103438340309550004199
0.0010954451150103438340309550004199
Yes, you could have set everything up as symbolic, then substitute the values of k,e,m,w in at the end.
syms k w m e s
eqn = (w*sqrt(e*m/2)*sqrt(sqrt(1+(s/(w*e))^2)-1)) - k
eqn =
w*((s^2/(e^2*w^2) + 1)^(1/2) - 1)^(1/2)*((e*m)/2)^(1/2) - k
ssol = solve(eqn,s)
ssol =
-(2*k*(k^2 + e*m*w^2)^(1/2))/(m*w)
(2*k*(k^2 + e*m*w^2)^(1/2))/(m*w)
So then read the help for subs.
help subs

  2 Comments

Thanks but I've got these values on k w m e latter.... before that, I didnt know the values of k w e m, then I solved out s:
clc
clear all
syms k w m e s
eqn=k==[w*sqrt(e*m/2)*sqrt(sqrt(1+(s/(w*e))^2)-1)]
k=1; e=0.0006; m=2000; w=2*pi*10^6;
sol=solve(eqn,s)
so s was:
sol =
-(2*k*(k^2 + e*m*w^2)^(1/2))/(m*w)
(2*k*(k^2 + e*m*w^2)^(1/2))/(m*w)
But latter I've got values on k w m e, I knew it was easy to calculate the numeric value of s by hand or calculator but I was searching for direct method in Matlab. Something like str2num or eval but it did not work. Thank you anyway
But I showed you how to solve it directly, two different ways.
You can leave those variables as symbolic, then use solve as I did. Then use subs, as I told you to do, substituting in the values for k,e,m,w.
Or you can set those values as constants first, then solve for s. Either way works. So what is the problem?

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