I received a negative MATLAB response while inserting the following requests:
syms x y z
solve (x+y==z,x-2*y==z,[x,y],'ReturnConditions', false)
I wanted to get x, and y in terms of z but, alas, I got the following error:
??? Error using ==> char
Conversion to char from logical is not possible.
Error in ==> solve>getEqns at 160
vc = char(v);
Error in ==> solve at 84
[eqns,vars] = getEqns(varargin{:});
Any help please?
Try using a cell array to wrap x and y instead:
>> syms x y z
>> X = solve (x+y==z,x-2*y==z,{x,y},'ReturnConditions', false);
>> X.x
ans =
z
>> X.y
ans =
0
This was required for previous versions of MATLAB. However, I'm using R2015a currently and your code works for me. I can't replicate your error. You may be using a previous version.
I think i found the hit-back to my problem.
As for solving system of two equations and two variables, we use a matrix of inputs, and outputs must be sprecified.
b=[x+y-z;x-3*y-z;]
b =
x + y - z
x - 3*y - z
[e t]=solve(b,x,y)
e =
z
t =
0
Thnk you all.
Related
>> syms x v(x) w(x);
>> eq1 = 2*v + 3*w == 4;
>> eq2 = 5*v + 4*w == 3;
>> sol = solve([eq1,eq2],[v,w])
I tried to implement this code in MATLAB, but error flashes out as "The second argument must be a vector of symbolic variables." I have tried similar things in Python using SymPy, but never such error comes. How to correct out this?
Have a look at the help file for the multivariate case of solve and the example in the help file
openExample('symbolic/SolveMultivariateEquationsAndAssignOutputsToStructureExample')
Applied to your problem
syms v w;
eq1 = [2*v + 3*w == 4;5*v + 4*w == 3];
sol = solve(eq1)
sol.v
sol.w
but if you just want to solve for v w then you could use e.g.
[2 3;5 4]\[4;3]
I am trying to solve an equation for x in Matlab, but keep getting the error:
Empty sym: 0-by-1
The equation has an integral, where x is the upper bound and also part of the integrand 1. The code I use is the following:
a = 0.2; b= 10; c = -10; d = 15; mu = 3; sig = 1;
syms x t
eqn = 0 == a + b*normcdf(x,mu,sig)+c*int( normcdf(d + x - t,mu,sig)*normpdf(t,mu,sig),t,0,x);
A = vpasolve(eqn,x)
Any hints on where I am wrong?
I believe that the symbolic toolbox may not be good enough to solve that integral... Maybe some assume or some other trick can do the job, I personally could not find the way.
However, to test if this is solvable, I tried Wolfram Alpha. It gives a result, that you can use.
eq1=a + b*normcdf(x,mu,sig);
resint=c*(t^3*(d - t + x)*erfc((mu - x)/(sqrt(2)*sig)))/(4*sig*exp((-mu + x)^2/(2*sig^2))*sqrt(2*pi));
A=vpasolve(eq1+subs(resint,t,x)-subs(resint,t,0) ==0)
gives 1.285643225712432599485355373093 in my PC.
I'm trying to solve this equation in Matlab
dT=((-A-B*C+D*./E)
where C=sin(dT). dT is unknown. A, B, D, and E are known variables. Using Matlab's solve function:
Ans=solve(dT==((-gra-H_vap*m_lg+grb*./ro_cp),dT);
But I receive an error message. How do I solve this equation?
You haven't given us any specifics on the values of your known parameters, and I also believe that D*/E in your example were intended to be a more valid expression.
Anyway, here is an example of how you make use of the symbolic solver solve:
syms dT
A = 1
B = 2
D = [1 2]
E = [3 4]
eqn = -A - B*sin(dT) + D/E - dT == 0
soldT = solve(eqn,dT)
which produces the following output
% ...
eqn =
- dT - 2*sin(dT) - 14/25 == 0
% ...
soldT =
-0.18739659458654612052194305796251
See also the language docs for solve.
I've defined a function in Matlab like this:
clg = 2*pi*(alpha-alphai+0.5*A(1));
where A is an array which depends on alpha.
I want to evaluate clg for aplha=0.53.
I've tried subs(clg, alpha, 0.53), but It gives me a bunch of weird errors:
Error using sym/subs>normalize (line 210)
Entries in second argument must be scalar.
Error in sym/subs>mupadsubs (line 136)
[X2,Y2,symX,symY] = normalize(X,Y); %#ok
Error in sym/subs (line 124)
G = mupadsubs(F,X,Y);
Error in integral (line 45)
subs(clg, alpha, 0.53)
Any idea on how to achieve that?
Thanks!
I have no problem about your question.
>> syms x x1 x2
>> clg = 2*pi*(x-x1+0.5*x2);
>> subs(clg,x,0.53)
ans =
2*pi*(x2/2 - x1 + 53/100)
I am trying to reproduce the problem, but this works:
syms x
%A is a symbolic matrix that depends on x
A = inv([10*x, 2*x, 3*x;
4*x, 10, 6*x;
7*x, 8*x, 10*x])
%y is a symbolic expression defined such that
%it depends on A, but collapses to an expression of x
y = x + 0.5*A(2,2)
%subs returns a symbolic (in this case fraction) evaluation:
subs(y, x, 3)
%eval returns a numeric evaluation:
x = 3
eval(y)
(I have encountered the same error message as you in my own code, but have not yet hunted down its source.)
Please consider this example. I would like to solve x^3 - 2x > 0. I try the following commands:
syms x;
f = #(x) x^3-2*x;
solve(f(x)>0,x)
and Matlab returns this
ans = solve([0.0 < x^3 - 2.0*x], [x])
which is not what I expect. Therefore I use
solve(f(x)+x>x,x)
which returns
ans = Dom::Interval(2^(1/2), Inf) Dom::Interval(-2^(1/2), 0)
Can someone explain that why solve works successfully only in the second case?
Try adding the Real option to solve:
solve(f(x)>0,x,'Real',1)
ans =
Dom::Interval(2^(1/2), Inf)
Dom::Interval(-2^(1/2), 0)