Figure N4–2(4) If y′(c) = 0 and y′′(c) = 0, investigate the signs of y′ as x increases through c.
If y′(x) > 0 for x’s (just) less than c but y′(x) < 0 for x’s (just) greater than c,
then the situation is that indicated in Figure N4–3a, where the tangent lines
have been sketched as x increases through c; here c yields a local maximum.