7. For any RV X, show that P{X 0} inf{(t) : t 0}

7. For any RV X, show that P{X ≥ 0} ≤ inf{ϕ(t) : t ≥ 0} ≤ 1, where ϕ(t) = EetX , 0<ϕ(t)≤∞.