source("http://macosa.dima.unige.it/r.R")
f <- function(x,y) x*y
f1 <- function(x,y) f(x,y)-f(3,1)
PLANE(-3,4, -3,4)
CURVE(f,"brown"); CURVE(f1,"red"); POINT(3,1,"blue")
g <- function(x,y) (x-1)*(x+2*y+1/2)
g1 <- function(x,y) g(x,y)-g(2,-2)
CURVE(g,"seagreen"); CURVE(g1,"blue"); POINT(2,-2,"red")