x <- c(-5,-2.6,-0.4,2.2,3); y <- c(-1.3,-2.6,-1,4,5.4)
n <- length(x)
a <- sum(x); b <- sum(x^2); c <- sum(x^3); d <- sum(x^4)
e <- sum(y); f <- sum(x*y); g <- sum(x*x*y)
ma <- matrix(data = c(n,a,b,a,b,c,b,c,d), nrow = 3, ncol = 3)
noti <- matrix(data = c(e,f,g), nrow = 3, ncol = 1)
S <- solve(ma,noti); S
             [,1]
  [1,] -0.5020798
  [2,]  1.3559927
  [3,]  0.2361709
plot(x,y)
abline(h=seq(-2,5,1),v=seq(-4,2,1),lty=3)
F <- function(x) S[1]+S[2]*x+S[3]*x^2
plot(F,-5,3,add=TRUE,col="blue")

# Per approfondimenti vedi qui