Unknown variable within an equation

2 次查看（过去 30 天）
Shozeal
Shozeal 2018-10-3
评论： Shozeal 2018-10-5
Hello,
I have an equation of the form P*dawson(sqrt(log(P)-log(A)))== sqrt(B)*T to be solved.
Variable A and B are scalar, but variable T is a vector. I need to solve for P in the equation.
This is the code I have written
syms P
T=5:5:500;
A=2;
B=0.5;
eqn = P*dawson(sqrt(log(P)-log(A)))== sqrt(B).*T ;
s = solve(eqn,P)
s =
Empty sym: 0-by-1
That is the solution for s I got.
Please advise what can be done.
Thanks.

请先登录，再回答此问题。

采纳的回答

Star Strider
Star Strider 2018-10-4
Try this:
syms P T
A=2;
B=0.5;
eqn = P*dawson(sqrt(log(P)-log(A))) - sqrt(B).*T ; % Rearrange
eqnfcn = matlabFunction(eqn, 'Vars',{P,T}) % Anonymous Function
T=5:5:500;
for k1 = 1:numel(T)
Pv(k1) = fsolve(@(P)eqnfcn(P,T(k1)), 10);
end
figure
plot(T, Pv)
grid
xlabel('T')
ylabel('P')

更多回答（2 个）

Image Analyst
Image Analyst 2018-10-3
dawson needs to be defined also.
  2 个评论
Image Analyst
Image Analyst 2018-10-4
编辑：Image Analyst 2018-10-4
Olufemi's "Answer" moved here:
@Image analyst, sorry I don't get what you mean by Dawson needs to be defined. Dawson is a function in MATLAB. For example, you can have dawson(2) = 3.013403889237920e-01 when solved.
Thank you for the response.
Image Analyst
Image Analyst 2018-10-4
OK, it wasn't in my help. It must be in a toolbox that you have that I don't have.

请先登录，再进行评论。

Walter Roberson
Walter Roberson 2018-10-4
syms P
T=5:5:500;
A=2;
B=0.5;
s = arrayfun( @(t) vpasolve(P*dawson(sqrt(log(P)-log(sym(A))))== sqrt(sym(B)).*t, P), T);
plot(T, s)
You have to loop solving one value at a time. With the code you had, you were trying to find a single value for P that satisfied all of the equations at the same time.
  3 个评论
显示 1更早的评论
Walter Roberson
Walter Roberson 2018-10-4
You should not have changed the t to T on the line.
You appear to be using R2015a or earlier (I think it was... I would need to check), as a few releases ago arrayfun() started being able to handle sym variables.
syms P
T=5:5:500;
A=2;
B=0.5;
scell = arrayfun( @(t) vpasolve(P*dawson(sqrt(log(P)-log(sym(A))))== sqrt(sym(B)).*t, P), T, 'uniform', 0);
s = double([scell{:}]);
plot(T, s)
Nearly a straight line.
Shozeal
Shozeal 2018-10-5
Hi Walter, I actually use R2017a. So, after keeping the t and T in the expression as written above, I got something close to a straight line too. Though it takes a very long time to run if I should increase the range of T.
Thanks.

请先登录，再进行评论。

类别

在 Help Center 和 File Exchange 中查找有关 Mathematics 的更多信息

标签

产品

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by