First derivative test states that if f '(x) changes sign from positive to negative as x increases through c, then c is a point of local maxima, and f(c) is local maximum value.
Thus, if f '(x) changes its sign from positive to negative as x increases through c in the interval (c − h, c + h), then x = c is a point of local maximum.
If f '(x) changes its sign from positive to negative as x increases through c in the interval (c − h, c + h), then x = c is a point of ___local maximum___.