26. (a) Let f be differentiable on (a, b). Prove that if f '(x) 7E 0 for each x 6 (61,19), then f has at most one zero in (61,19). (b) Let f be twice differentiable on (a, b). Prove that if f\"(x) 75 0 for each x e (a, b), thenf has at most two zeros in (a, b)