1Change of sign of dydx occurs only due to (x-b)2n+1 cause of its odd power. So at b, dydx changes sign from -ve on the left to +ve on the right. So b is a point of minima. a is a pt. of extremum. it is neither min. nor max. since no change of sign. Here the curve makes 0 slope for a moment but doesn't create convexity or concavity .
