Remark The converse of above theorem need not
be true, that is, a point at which the derivative vanishes
need not be a point of local maxima or local minima For example, if f (x) = x3, then f ′(x) = 3x2 and so
f ′(0) = 0 But 0 is neither a point of local maxima nor
a point of local minima (Fig 6 13)