How to find the value of a variable in MATLAB that exists on both sides of th eequal sign?

Does anybody know how we can find the value of a variable in MATLAB that exists on both sides of equal sign, e.g. n in 2n+q=n+r (n is the only variable)?

In this case it is so simple; n=r-q. But what if the equation is very complex and we are not able to transfer the variable to one side?

Answer 1

[Star Strider]

If you have the Symbolic Math Toolbox:

syms n q r
Eq = 2*n+q == n+r 
n = solve(Eq, n)

produces:

n =

      r - q

To solve them numerically for n, make the entire equation equal to zero and use the fzero function:

r = 3;
q = 5;

f = @(n) [2*n+q - (n+r)];

n = fzero(f, 1)

produces:

n =

      -2

[Kourosh]

Really helpful, thanks mate

[Star Strider]

My pleasure!

[Kourosh]

Dear Star,

I have one specific problem in my case. What you mentioned is correct. But in my case with the funntion

su=1; b=0.5; l=1.571; p=0.16; t=atan(2*pi()/p); Landa=-tan((pi()/2)-t);

pn=p/l; B=asin(pn);

q=0.49*pn^-0.55; r=0.76*pn^-0.55;

Nmax=4.266*cos (B)*((b/l)/0.318)*su*l^2; Tmax=0.191*((b/l)/0.318)*su*l^3;

f = @(Nn) [((Nn/Nmax)^(q-1))/(1-(Nn/Nmax)^q)^((r-1)/r)-(r*Landa*Nmax/(q*Tmax))];

Nn = fzero(f, 0)

where I know the result is definitely between 0 and 10, the result by Matlab is NaN.

I am pretty sure the equation is correct. Is there any other way to solve it?

Cheers, Cyrus

[Star Strider]

I don’t know what the equation is, but plotting it:

Nn = linspace(-10,10);
figure(1)
plot(Nn, real(f(Nn)))
grid

shows that the real part doesn’t have a zero crossing anywhere in the region. (It has a small region in the Nn domain where it generates complex values.)

Answer 2

[Brian B]

If you have the Symbolic Toolbox, you can use:

>> syms n q r

>> solve('2*n+q=n+r','n')
ans =
r - q

[Kourosh]

Thanks mate