Symbolic rewritten in matlab

Question

Miraboreasu 2022-6-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1745085-symbolic-rewritten-in-matlab

评论： Walter Roberson 2022-6-21

alpha=log(r)/t0/(r-1);
beta=r*log(r)/t0/(r-1);
p0=p*(exp(-alpha*t)-exp(-beta*t))/(exp(-alpha*t0)-exp(-beta*t0));
%r=?
%I tried this, plug alpha and beta with the expressions
syms r t p p0 t0
alpha=log(r)/t0/(r-1);
beta=r*log(r)/t0/(r-1);
%p0=p*(exp(-(log(r)/t0/(r-1))*t)-exp(-(r*log(r)/t0/(r-1))*t))/(exp(-(log(r)/t0/(r-1))*t0)-exp(-(r*log(r)/t0/(r-1))*t0));
eqn=(p*(exp(-(log(r)/t0/(r-1))*t)-exp(-(r*log(r)/t0/(r-1))*t))/(exp(-(log(r)/t0/(r-1))*t0)-exp(-(r*log(r)/t0/(r-1))*t0))==p0);
r=solve(eqn,[t, p, p0 t0])
%It doesn't give what I want

Hello, I have a equation, p0= above

I want to rewrite it as r=something

how can I do this in MATLAB or any other website?

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Walter Roberson 2022-6-21

在 MATLAB Online 中打开

You cannot solve one equation for four variables. You should be trying

solve(eqn, r)

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Answer 1

Star Strider 2022-6-21

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1745085-symbolic-rewritten-in-matlab#answer_990355

在 MATLAB Online 中打开

An analytic solution is likely not possible, due to the nature of the expression.

The only option is to solve it numerically for ‘r’ given appropriate values for the other variables —

syms p p0 r t0 t

alpha=log(r)/t0/(r-1);

beta=r*log(r)/t0/(r-1);

Eqn = p0 == p*(exp(-alpha*t)-exp(-beta*t))/(exp(-alpha*t0)-exp(-beta*t0))

Eqn =

p0fcn = matlabFunction(rhs(Eqn)-p0)

p0fcn = function_handle with value:

@(p,p0,r,t,t0)-p0-(p.*(exp(-(t.*log(r))./(t0.*(r-1.0)))-exp(-(r.*t.*log(r))./(t0.*(r-1.0)))))./(exp(-(r.*log(r))./(r-1.0))-exp(-log(r)./(r-1.0)))

p = rand;

p0 = rand;

t = rand;

t0 = rand;

r = fsolve(@(r)p0fcn(p,p0,r,t,t0), rand)

No solution found. fsolve stopped because the relative size of the current step is less than the value of the step size tolerance squared, but the vector of function values is not near zero as measured by the value of the function tolerance.

r = -0.0174

.