Let X₁,X₂, . . . , X_n be a random sample of size n from a geometric distribution that has pmf f(x; θ) = (1 − θ)^xθ, x = 0, 1, 2, . . . , 0 < θ < 1, zero elsewhere. Show that Σⁿ₁ X_i is a sufficient statistic for θ.