SOLVE with symbolic and non symbolic variables

Hello everybody!

I would like to solve a system with 5 symbolic-variables. The muneric values in the ecuación are also variables that come from other calculations.

I made this way...

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 as=0.4
 ds=0.04318
 n=2 
 j=0.9
 nu_3=0.25
 epsilon_st=1.15

% This are the values that come from other calculations and I should use them in the system.

 syms y_a Eta a_sw f_eta

 eq1=sym('y_a=0.5 *(as-Eta*(as-ds))')

 eq2=sym('a_sw=(2*y_a/sind(180/n))*(sqrt((1-2*y_a/as)/(2*y_a/as))/atand(sqrt((1-2*y_a/as)/(2*y_a/as))))')

 eq3=sym('f_eta=as*nu_3/a_sw')

 eq4=sym('j=sqrt((Eta^3+epsilon_st*Eta)/((1+epsilon_st)*f_eta))')

 eq5=sym('as*nu_3=ds/sind(180/n)*sqrt(as/ds-1)/atand(sqrt(as/ds-1))')

 Solucion = solve(eq1,eq2,eq3,eq4,eq5)

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...and I am getting this prolblem:

Warning: Explicit solution could not be found. > In solve at 160

Should I define the problem in other way? Somebody Knows what am I doing not rigth?

Thanks a lot!, David

[Azzi Abdelmalek]

what are your unknowns variales, it seems that you have 4 four 5 equations

Answer 1

[Star Strider]

You need to make some changes in your code.

First, change all the sind and atand to sin and atan. The Symbolic Math Toolbox doesn't recognize sind and atand.

Second, I suggest changing the single equals (=) to double equals (==) in your symbolic equations.

When I did this:

   syms y_a Eta a_sw f_eta 
   eq1=sym('y_a==0.5 *(as-Eta*(as-ds))') 
   eq2=sym('a_sw==(2*y_a/sin(180/n))*(sqrt((1-2*y_a/as)/(2*y_a/as))/atan(sqrt((1-2*y_a/as)/(2*y_a/as))))')  
   eq3=sym('f_eta==as*nu_3/a_sw') 
   eq4=sym('j==sqrt((Eta^3+epsilon_st*Eta)/((1+epsilon_st)*f_eta))') 
   eq5=sym('as*nu_3==ds/sin(180/n)*sqrt(as/ds-1)/atan(sqrt(as/ds-1))') 
   Solucion = solve(eq1,eq2,eq3,eq4,eq5)

I was able to get expressions for:

Solucion.f_eta
Solucion.j
Solucion.n
Solucion.nu_3
Solucion.y_a

You will have to do subs or eval to get your constants into your equations, then do simplify(collect(expand(...))) on the elements of ‘Solucion’ to make them readable.