source("http://macosa.dima.unige.it/r.R")  # salta se già usata
f <- function(x) x^2
a <- 1.6; graph1F(f,-a,a,"brown")
POINT(0, 1/4, "seagreen")
for (x in seq(1/4,3/2,1/4)) {
polyl(c(0,x,x),c(1/4,f(x),5),"seagreen"); POINT(x,f(x),"seagreen") }