Introducing a Variable in simple equation?

Why is simple code is not working? I am a novice here.

Qc = 40;
Qr = 55;
I = 78;
Qs = 100;
I = sqrt((Qc + Qr -Qs)/R_Ts);
disp(R_Ts)

It is showing Undefined function or variable 'R_Ts'.

Answer 1

[John D'Errico]

You have not defined that variable. How does MATLAB know what it is? You defined all the other variables, but not that one.

If you have the symbolic toolbox, then do this first:

syms R_Ts

Now, it appears as if you want MATLAB to know that you wish to solve for the unknown variable. Computer programs don't read your mind. They do what you tell them to do.

Qc = 40;
Qr = 55;
I = 78;
Qs = 100;

This next line creates an equation of the equality as you posed it, with the unknown variable R_Ts in it.

EQ = I == sqrt((Qc + Qr -Qs)/R_Ts);

And now we apply solve.

R_Ts = solve(EQ,R_Ts)
-5/6084

Or, convert that to a numeric form:

vpa(R_Ts)
ans =
-0.00082182774490466798159105851413544

double(R_Ts)
ans =
     -0.000821827744904668

[Mahbubur Rahman]

Thanks a lot for the reply. But what if there are two equations containing the R_Ts, like this -

Qr = 55;
I = 78;
Qs = 100;
Qc = 45*I - ((R_Ts)^2);
I = sqrt((Qc + Qr -Qs)/R_Ts);
disp(R_Ts)

What to do this stage? And why the disp does not work in these cases?

[Walter Roberson]

You would still need the

 syms R_Ts

before you use R_Ts, and you still need the solve(),

syms R_Ts
Qr = 55;
I = 78;
Qs = 100;
Qc = 45*I - ((R_Ts)^2);
Eq = I == sqrt((Qc + Qr -Qs)/R_Ts)
R_Ts = solve(EQ,R_Ts)

sol =