function [y, err] = regressao_linear(xp, yp, x, tol)
 y=''
 if size(xp)(2)==0 then
  err='Erro de entrada: quantidade de pontos é 0'
  return
 end
 if size(xp)~=size(yp) then
  err='Erro de entrada: vetores de tamanhos diferentes'
  return
 end
 if tol<0 then
  err='Erro de entrada: tolerancia < 0'
  return
 end
 for i=1:size(xp)(2)
  for j=i+1:size(xp)(2)
   if xp(i)==xp(j) then
    err='Erro de entrada: existem pontos com o mesmo x'
    return
   end
  end
 end
 mediax=0
 mediay=0
 for i=1:size(xp)(2)
  mediax=mediax+xp(i)/(size(xp)(2))
  mediay=mediay+yp(i)/(size(yp)(2))
 end
 for i=1:size(xp)(2)
  b=(xp(i)*(yp(i)-mediay))/(xp(i)*(xp(i)-mediax))
 end
 a=mediay-b*mediax
 y=a+b*x
 err=''
endfunction

function y = calcula(f,x)
 y=evstr(f)
endfunction

function [A, b, err] = montar_sistema(xp, yp, g, tol)
 //A*x=b
 for i=1:size(g)(2)
  for j=1:size(g)(2)
   temp=0
   for k=1:size(xp)(2)
     temp=temp+calcula(g(i),xp(k))*calcula(g(j),xp(k))
   end
    A(i)(j)=temp
  end
 end
 for i=1:size(g)(2)
  temp=0
  for j=1:size(yp)(2)
   temp=temp+yp(j)*calcula(g(i),xp(j))
  end
  b(i)=temp
 end
 x=inv(A)*b
 disp(x)
 err=''
endfunction

[A, b, err]=montar_sistema([2 3 5], [7 13 31], ['x' 'x^2' '2'], 0)

[y, err]=regressao_linear([1 5 3], [4.3 20.3 12.3], 4, 0.001)