5 Question /raag [9] Prove that if f(x) > 0 is continuous on the interval [a, b], differentiable on the interval (a, b) and rotated about the x-axis, a surface of revolution is formed with outer area given by Bewys dat indien f(x) > 0 kontinu is op die interval [a, b], differensieerbaar op die interval (a, b) en roteer word om die x-as, 'n omwentelingsoppervlak met oppervlakte gevorm word waarvan die buite-oppervlakte gegee word deur b 2nf(x) If' (x) 12 + 1 dx. a 217 +11 4