11-20 (a) Find the intervals on which f is increasing or decreasing. (b) Find the local maximum and minimum values of f. (c) Find the intervals of concavity and the inflection points. 11. f(x) = 2x* + 3x' - 36x 12. f(x) = 4x3+ 3x' - 6x + 1 13. f(x) = x4 - 2x' + 3 14. f(x) = x' x- + 3 15. f(x) = sin x + cos x, Ox $ 2TT 16. f(x) = cos-x - 2 sin x, Ox $ 2n 17. f (x) = e"* + ex 18. f(x) = x' In x 19. f(x) = (In x) / vx 20. f (x) = Vxe-x