Warning: Explicit solution could not be found. But in the command line it works!

Hello,

I have this .m program:

clear
syms C1 C2 x;
w=2.*pi.*4E8;
L=84.4E-9;
C=32.22E-12;
r=5;
Z0=50;
Zload=r+1i*(L*w-(1/(C*w)));
Zin=Z0*((Zload+1i*Z0*tan(x*2*pi))/(Z0+1i*Zload*tan(x*2*pi)));
ZC1=1./(1i.*C1.*w);
ZC2=1./(1i.*C2.*w);
Z_eq=Zin*ZC2/(Zin+ZC2)
C2_sol=solve('Zeq-50=0',C2);

with the output:

Z_eq =

-(104857600*i*((878603275321697*i)/4398046511104 + 50*i*tan(2*pi*x) + 5))/(5270717853328913*C2*(tan(2*pi*x)*(5*i - 878603275321697/4398046511104) + 50)*((50*((878603275321697*i)/4398046511104 + 50*i*tan(2*pi*x) + 5))/(tan(2*pi*x)*(5*i - 878603275321697/4398046511104) + 50) - (2097152*i)/(5270717853328913*C2)))

Warning: Explicit solution could not be found. 
> In solve at 170
  In program_name at 13
[ empty sym ]

So, the solve command from the last line doesn't work. But if I copy the Z_eq output and paste it to solve the equation in the command line:

>> solve ('-(104857600*i*((878603275321697*i)/4398046511104 + 50*i*tan(2*pi*x) + 5))/(5270717853328913*C2*(tan(2*pi*x)*(5*i - 878603275321697/4398046511104) + 50)*((50*((878603275321697*i)/4398046511104 + 50*i*tan(2*pi*x) + 5))/(tan(2*pi*x)*(5*i - 878603275321697/4398046511104) + 50) - (2097152*i)/(5270717853328913*C2)))-50=0', C2)

ans =

((41505174165846491136*i)/26353589266644565 - tan(2*pi*x)*((921282308023723753472*i)/131767946333222825 - 41505174165846491136/26353589266644565) + 921282308023723753472/131767946333222825)/(878603275321697*i + 219902325555200*i*tan(2*pi*x) + 21990232555520)

Do I do something wrong? Is there an option which I forgot to mention in the .m program?

Thanks!

Answer 1

[Andrew Newell]

Change the last line to

C2_sol=solve(Z_eq-50,C2);

I think the problem is that solve determines what the symbolic variables are using symvar, and this is the output:

>> symvar('Z_eq-50=0')

ans =

    'Z_eq'

In other words, it doesn't look inside Z_eq to find C2.

[Ionesco]

Thank you for the magic. It works!

Have a great day!