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")