TD 4 - Probas Stats Hakim ABDOUROIHAMANE 18 Octobre 2019 Exercice 1 Calculer les fonctions génératrices des lois usuelles : Bernouilli, binomiale, géométrique, Poisson. En déduire leur moyenne et leur variance. E(X) = G0x (1) +∞ P E(X) = x × P (X = x) x=0 +∞ P E[f (X)] = i × P [f (X) = i] i=0 Exemple P (X = 1) = P (X = 3) = 12 P (X = i) = 0 si i ∈ / 1,3 f (1) = 6 et f (3) = 14 f (x) = 6 → X = 1 donc P (f (X) = 6) = 12 de même : P [f (X) = 14] = f (X = 3) = E[f (x)] = 12 × 6 + 12 × 14 1 2 • X suit la loi de Bernouilli de paramètre p ↔P(X = 1) = petP(X = 0) = (1-p) +∞ P Gx (s) = E(sX ) = P (X = i) × si x=0 Gx (s) = P (X = 0) × s0 + P (X = 1) × s1 Gx (s) = (1 − p) × 1 + p × s Gx (s) = 1 + p(s − 1) G0x (s) = p E(X) = G0 x(1) = p V (X) = E(X 2 ) − E(X)2 V (X) = E[X(X − 1)] + E(X) − E(X)2 V (X) = G00X (1) + G0X (1) + G0X (1)2 V (X) = 0 + p − p2 = p(1 − p) 1 • X(n, p) ↔P(X +∞ P Gx (s) = E(sX ) = P (X = i) × si i=0 GX (s) = +∞ P i=0 Complément de formules – Formule 1 : Si a ∈ R, a 6= 1 et n ∈ N n P ( n+1 a2 = 1−a 1−a i=0 – Formule 2 : Soit a ∈ R, ea = +∞ P = i=0 ai i! – Corollaire 1 : Si a 6= 0 et |a| < 1 +∞ n P i P 1 a = limn→+∞ ai = 1−a i=0 i=0 – Corollaire 2 : n P ai = i=k ak −an+1 1−a – Loi géométrique Si X tilde G(p) alors X prend ses valeurs dans N* P (X = i) = (1 − p)i−1 × p +∞ P GX (s) = E(sX ) = P (X = i) × si GX (s) = 0 × s0 + i=0 +∞ P P (X = i) × si i=1 GX (s) = +∞ P +∞ P i=1 i=1 (1 − p)i−1 × p × si = p × GX (s) = p × s +∞ P (1 − p)i−1 × si [(1 − p)i−1 × si−1 ] i=1 En posant j = i − 1, on voit que j est un indice de sommation allant de 0 à +∞ donc : +∞ P GX (s) = ps × [(1 − p) × s]j j=0 1 GX (s) = ps × 1−(1−p)s ps GX (s) = 1−(1−p)s – Loi de Poisson : X tilde Poisson(theta) i P (X = i) = e−Θ × Θi! +∞ P GX (s) = E(S X ) = P (X = i)si i=0 GX (s) = +∞ P i=0 e−Θ × Θi i! × si 2 GX (s) = e−Θ × +∞ P i=0 (Θ×s)i i! GX (s) = e−Θ × eΘ×s GX (s) = eΘ×s−Θ GX (s) = eΘ×(s−1) G0X (1) = Θ G00X (1) = Θ2 × eΘ×(s−1) V (X) = Θ Exercice 2 1. 9! 2. P (X = 0) = 3. P (X = 1) = 4. P (X = 2) = 5. P (X = 3) = 6. P (X = 4) = 7. 4 P x=0 5×8! 70 9! = 126 4×5! 33 9! = 126 13 4×3×5×6! = 126 9! 4×3×2×5×5! 5 = 126 9! 4!×5! 1 9! = 126 P (X = x) × x = 84 126 = 2 3 3