a) If Sₙ= Σⁿₖ₌₀|zn| is bounded for each n ∈ N, show that no Σ[infinity]ₖ₌₀ converges absolutely. b) If {aₙ} is a decreasing and bounded sequence of non-negative real numbers, then Σ[infinity]ₖ₌₀ (aₙ - aₙ₊₁) converges absolutely