Empty sym: 0-by-1

Hello, I'm new to matlab so any help would be appreciated! I needed matlab to solve some equation systems for my finite element class.

The method I used to use worked until this 4x4 system. I'm not sure what's the best way to solve this and with my way i get "Empty sym: 0-by-1" as an answer.

(the "P(u3)" is just a symbol, not a multiplication. P(u3)=p....and also E*A/L=5*10^7)

syms u2 v2 u3 p
e1= (5*10^7)*(2/4*u2 - 1/4*u3 + 3/4*u3)==0;
e2= (5*10^7)*(6/4*v2+sqrt(3)/4*u3-3*sqrt(3)/4*u3)==19613;
e3= (5*10^7)*(-1/4*u2+sqrt(3)/4*v2+1/4*u3-3/4*u3)==p;
e4= (5*10^7)*(sqrt(3)/4*u2-3/4*v2-sqrt(3)/4*u3+3*sqrt(3)/4*u3)==sqrt(3)*p;
result = solve(e1,e2,e3,e4)

result = 

  struct with fields:

     p: [0×1 sym]
    u2: [0×1 sym]
    u3: [0×1 sym]
    v2: [0×1 sym]

vpa(result.p)
 
ans =
 
Empty sym: 0-by-1

Answer 1

[Star Strider]

Your error was in not leaving it in the original matrix form.

This works, however I am confused by your notation, so this may need to be corrected:

syms u2 v2 u3 p

Mtx = 5E+7 * [2/4    0   -1/4  +3/4;
              0    6/4    sqrt(3)/4  -3*sqrt(3)/4;
            -1/4 +sqrt(3)/4   +1/4  -3/4;
            sqrt(3)/4  -3/4  -sqrt(3)/4  +3*sqrt(3)/4];

Vct = [0;  19613;  p; sqrt(3)*p];
V = [u2;  v2;  u3; p];

S = vpasolve(Mtx * V == Vct);
Sp = S.p
Su2 = S.u2
Su3 = S.u3
Sv2 = S.v2

producing:

Sp =
-5661.785414808065059658960697873
 
Su2 =
17115934194006.985529996788305132
 
Su3 =
34231868371028.614815569381431287
 
Sv2 =
-9881889214341.853390449609171597

.

[Nikos Gavalakis]

Thank you very much m8!! I'm going to solve matrices this way from now on.

Btw I might have done something else wrong because i wasnt expecting u2,v2,u3 to be such large numbers :(

[Star Strider]

As always, my pleasure!

Since the matrix is multiplied by 5E+7, the large numbers do not surpise me.