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vpasolve and solve return empty sym 0-by-1, SOLUTION DOES EXIST

Asked by Bob photonics on 25 May 2018
Latest activity Commented on by Bob photonics on 15 Jun 2018

When I run this code I get empty strings for when setting a range for vpasolve, when I do not set the range I only get one solution, even with random on. The range is set so that it does include the one solution matlab gives me, x=2, y=0 and b=1.5. for the range I've tried -Inf Inf and NaN NaN, to have it try all numbers. So please do not give me the answer of saying that my system has no solution, it clearly does. It also won't solve it symbolicly (same issue with solve), while I can give 1 possible solution. 0.5*(x+y)=1, b*(x+y)=3 ---> x+y=2 and b=1.5

So something else must be wrong, I would appreciate it if you let me know what I'm doing wrong here.

clear all; %just to be safe
syms x y b 
a=0.5;
somevalue=1;
someothervalue=3;
eq1= a*x+a*y == somevalue; %this is your first equation
eq2= b*x+b*y == someothervalue; %this is your 2nd equation
eqs=[eq1,eq2]; %use this for vpasolve and set range in range
vars=[x,y,b]; %these are the variable you want to solve for
range = [-1 3; -2 5; -Inf Inf]; %NaN means you set no range
%you can use solve or vpasolve, second one being numeric, which is the one you'll probably want
sol=zeros(5,3);
for i = 1:5 
    temp1 = vpasolve(eqs, vars, range, 'random', true);
    temp = solve(eqs, vars);
    sol(i,1) = temp.x;
    sol(i,2) = temp.y;
    sol(i,3) = temp.b;
end
sol
temp1.x 
temp1.y
temp1.b

Now I have another clear problem/error when using the solve option, obviously the answer here should be 9:

syms x
eq12 = -3 == sqrt(x);
solve(eq12)
ans =
Empty sym: 0-by-1

  1 Comment

Still waiting for an answer to the first part, the first piece of code.

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1 Answer

Answer by Walter Roberson
on 27 May 2018

sqrt(x) is short-hand for x^(1/2) . 9^(1/2) only refers to the primary square root, not to all square roots. Work-around:

>> syms A
>> solve(A==sqrt(x),x)
ans =
A^2
>> subs(ans,A,-3)
ans =
9

  9 Comments

I just tried this, and it gives me an error that it cannot find an explicit solution.

clear all; %just to be safe
syms x y b 
a=0.5;
somevalue=1;
someothervalue=3;
eq1= a*x+a*y == somevalue; %this is your first equation
eq2= b*x+b*y == someothervalue; %this is your 2nd equation
%Boundary condition for x and y
cd1= x >= -1;
cd2= x <= 3;
cd3= y >= -2;
cd4= y <= 5;
eqconds=[eq1,eq2,cd1,cd2,cd3,cd4];
vars=[x,y,b]; %these are the variable you want to solve for
temp = solve(eqconds, vars);
Warning: Cannot find explicit solution. 
> In solve (line 316) 

Of course for vpasolve we get

temp = vpasolve(eqconds, vars)
temp = 
      x: [0x1 sym]
      y: [0x1 sym]
      b: [0x1 sym]

Sorry, I do not know. I recommend opening a support case.

Thank you very much for all the help and I'll do that. The help is definitely appreciated ;)

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