Show that (a) tanx³ = o(x²) as x→0. (b) x = o(sqrt(x)) as x→0. (c) O(1) = o(x) as x→infinity. (d) f(x)g(x) = x⁻¹ + O(1) as x→0, where f(x) = x + o(x²), g(x) = x⁻² + O(x⁻¹).