In Problems 17 and 18, f(x ) is continuous on (-00, co ) and has critical numbers at x = a, b, c, and d. Use the sign chart for f' (x ) to determine whether f has a local maximum, a local mini- mum, or neither at each critical number. 17. f' (x) + + + 0 ND- - - ND + + + 0 +++