3 sequential equations doesn't solve.. some help plz

the three equations are on the image

for this I wrote the following code:

>> syms vth vs id

>> eq1=-vth+0.5+0.4*((0.6+vs)^0.5-0.6^0.5);

>> eq2=-id+0.5*9*10^-4*(2.5-vs-vth)^2;

>> eq3=-vs+2*1000*id;

>> S=solve(eq1,eq2,eq3,vth,vs,id)

but there comes the error:

*??? Error using ==> mupadmex Error in MuPAD command: symbolic parameters not allowed in non-polynomial equations [numeric::solve]

Error in ==> sym.sym>sym.mupadmexnout at 2003 out = mupadmex(fcn,args{:});

Error in ==> solve at 76 [symvars,R] = mupadmexnout('mllib::solvefull',eqns,vars);*

--> i don't know what i've done wrong. my MATLAB version is 7.10.0(R2010a)

p.s. as for the code, I was trying to get a matrix S which contains the solutions of the three equations and I would've obtained the individual solutions by S.vth, S.id, S.vs

anyway... whats the problem here???

Answer 1

[Walter Roberson]

There are two solutions,

id = 0.173281035908099e-2, vs = 3.46562071816199, vth = .996696709643063

id = 0.426827703050411e-3, vs = .853655406100821, vth = .672431863201621

As to the problem: it appears to me you might have hit a limitation in your version of MuPAD. The square roots seem to be throwing it off. I do not have MuPAD to test with, so I do not know if there is a symbolic work-around.

[chul hyun kwag]

thanks for doing the calculation for me ^^ i think you're right there's something wrong with my mupad or something..

but i eventually found out a way to solve this by directly using the mupad toolbox. Still don't understand why my MATLAB's Mupad properly doesn't work though

Answer 2

[chul hyun kwag]

For anyone who is stuck with a similar problem with me, here's how I eventually got over this error. As Roberson answered, I guess there is something wrong with my license or MATLAB program but here's my way of doing it anyway:

1. Click the 'Start' button on the bottom left corner

2. Start-Toolboxes-Symbolic Math-MuPAD

3. Click 'New Notebook' in order to type in code

4. you'll get a blank notebook

5. then I typed in a "solve()" function. the code is as follows: >>solve([-x+0.5+0.4*((0.6+y)^0.5-0.6^0.5)=0,-z+0.5*9*10^-4*(2.5-y-x)^2=0,-y+2*1000*z=0],[x,y,z])

**the codes appear in red. make sure to write in the line with the grey colored '['.

I'm not that good with this MuPAD so i typed in every equation by hand. In MuPAD, it seems that you can't use solve() like solve(eq1,eq2,eq3,var1,var2,var3). Instead do it like: solve([eq1=0,eq2=0,eq3=0],[x,y,z]) . Just like a matrix. that's what I've done.

[Andrei Bobrov]

>> syms x y z
eq1 = -x+0.5+0.4*((0.6+y)^0.5-0.6^0.5);
eq2 = -z+0.5*9*10^-4*(2.5-y-x)^2;
eq3 = -y+2*1000*z;
roots1 = solve(eq1,eq2,eq3,x, y, z)
roots1 = 
    x: [4x1 sym]
    y: [4x1 sym]
    z: [4x1 sym]