fx <- expression(3*x^2/2+x+log(x+2)/5-log(3-x)/5); D(fx,"x")
  3 * (2 * x)/2 + 1 + 1/(x + 2)/5 + 1/(3 - x)/5
f1 <- function(x) 3 * (2 * x)/2 + 1 + 1/(x + 2)/5 + 1/(3 - x)/5
plot(f1,-5,5,ylim=c(-20,20),n=10000,type="p",pch=".", col="blue")
f2 <- function(x) (3*x^3-2*x^2-19*x-7)/(x^2-x-6)
plot(f2,-5,5,n=10000,type="p",pch=".",col="red",add=TRUE)
  
# Ovvero:
source("http://macosa.dima.unige.it/r.R")
f=function(x) 3*x^2/2+x+log(x+2)/5-log(3-x)/5; deriv(f,"x")
  3 * (2 * x)/2 + 1 + 1/(x + 2)/5 + 1/(3 - x)/5
f1 = function(x) 3 * (2 * x)/2 + 1 + 1/(x + 2)/5 + 1/(3 - x)/5
f2 = function(x) (3*x^3-2*x^2-19*x-7)/(x^2-x-6)
Plane(-5,5, -20,20); graph2(f1, -5,5, "red")
Plane(-5,5, -20,20); graph2(f2, -5,5, "blue")