I am having problems using syms

Wrote a programe in Matlab2009b to solve a what seemed to be a simple physical problem. Now I am having trouble with syms x real, getting the same error over and over again:

??? Error using ==> mupadmex
Error in MuPAD command: DOUBLE cannot convert the input expression
into a double array.

If the input expression contains a symbolic variable, use the VPA
function instead.

Error in ==> sym.sym>sym.double at 927
            Xstr = mupadmex('mllib::double', S.s, 0);

Error in ==> sym.sym>privformatscalar at 2539
    x = double(x);

Error in ==> sym.sym>privformat at 2524
    s = privformatscalar(x);

Error in ==> sym.sym>sym.subsasgn at 1418
                [inds{k},refs{k}] = privformat(inds{k});

Error in ==> mnm1 at 8
vx(x)=sqrt(g.*(x^2*((1+u)/Lv)-2.*u.*x-b^2.*((1+u)/Lv)+2.*u.*b));

And the code is:

clc;clear all;format compact
Lv=6;
Lt=11;
b=4;
u=0.15;
g=9.81;
syms x real
vx(x)=sqrt(g*(x^2*((1+u)/Lv)-2*u*x-b^2*((1+u)/Lv)+2*u*b));
ax(x)=g.*(x-u*(Lv-x))/Lv;
dx=b:0.01:Lv;
a=ax(dx);
v=vx(dx);
vm=vx(dx);
hold on
plot(dx,a,'b')
plot(dx,v,'r')
ax(x)=g;
vx(x)=sqrt(2.*(x-Lv).*g+vm.^2);
dx=Lv:0.01:Lt;
a=ax(dx);
v=vx(dx);
plot(dx,a,'b');legend('a(x)','Location','best')
plot(dx,v,'r');legend('a(x)','Location','best','v(x)','Location','best')
xlabel('distance x[m]');ylabel('acceleration a[m/s^2]/velocity v[m/s]');

If I try to input numerical value for x for example x=(0:0.1:Lt), as this would be accurate enough and would also provide the correct interval, the error I get then is that matrix must square. I did also use

   which -all syms

and I have the syms toolbox installed, so this does not seem to be a problem.

I am also fairly new to matlab and this seems to be a dead end for me. All help will be appreciated, thank you.