How to solve symbolic equation in matlab

Question

I have an equation in Matlab according to X parameter . I want to find the amount of X for the random amounts of F(x) . and I tried the code below . but It gives me two different results while my equation should have just one result .

even I tried the roots(f) instead of solve(f) but it gave me an error :

??? Undefined function or method 'isfinite' for input arguments of type 'sym'.

anybody can help me in this ? what should I do ? even if I have a wrong idea about solving this problem please tell me . Thank you

function betaDistribution_2(a,b)

    syms  x ;
    y=inline((x^(a-1))*((1-x)^(b-1)));
    beta=quad(y,0,1);
    g=(1/beta)*(x^(a-1))*((1-x)^(b-1));
    % I have this equation and I want to find the amount of x for the random 
    %amounts of p 
    p=int(g,x,0,x); 

    for i=0:50
       fxi=rand(1);
       f=p-fxi;
       xi=solve(f);
       result=eval(xi);
       disp(result)
    end

    end

Answer 1

Try to filter your solutions.

a=1;
b=2;
% a threshold for imagery part
SMALL=1e-9;

syms x real;
y=inline((x^(a-1))*((1-x)^(b-1)));
beta=quad(y,0,1);
g=(1/beta)*(x^(a-1))*((1-x)^(b-1));
p=int(g,x,0,x);

% return true for physically meaningfull results
filter = @(xc) abs(imag(xc))<SMALL && real(xc)>0 && subs(g, x, xc) > 0 && subs(p, x, xc)>0;

for m=0:50
  fxi=rand(1);
  f=p-fxi;
  xi=solve(f, x);
  result=eval(xi);

  idx = arrayfun (filter, result);
  result = result(idx);
  % make sure it is OK
  assert(length(result)==1);
  disp(result)
end