QUESTION 4 Let f (x) =_ Find the interval ( a. b ) where f increases. As your answer please input a + b. X x2 QUESTION 5 Let f (x) =x3- 1 1x2+ 39x- 47. Find an open interval where f decreases. As your answer please input ( + in decimal form with three significant digits after the decimal point. QUESTION 6 Let f (x) = (x-2) x2/3. Find the interval (a, b) where f decreases. As your answer please input atb