f(x)=(x+1)/ (x (x+4))
f(x)=x/(x^2-4)
f(x)=(3x+3)/(2x+4)
f(x)=(x^3-1)/(x^2-9)
f(x)=3 / ( x^2-4 )
f(x)=(x^4+x^2+1)/(x^2-1)
F(X)=x^2/(x^2+x-6)
