# trazimo gde je D(Xn) najmanja

s2 = 100

D_xn = function(N,n) {
  (N-n)*s2/(N*n)
}

D_xn_1 = D_xn(4000,400)
D_xn_2 = D_xn(300,30)
D_xn_3 = D_xn(300000000,3000)
D = c(D_xn_1, D_xn_2, D_xn_3)
which.min(D)