$\displaystyle (a) \frac{x^2}{1 + x^4}$ over $\displaystyle (-\infty,\infty).$
$\displaystyle (b) \frac{\ln(x)}{x}$ over $\displaystyle [1,\infty).$
Try them out and you'll see. For $\displaystyle (a)$ the criticals are 0,1 and -1, and as $\displaystyle x \rightarrow \infty, f(x) \rightarrow 0,$ so might this be a minimum? Also, +-1 seem to be a single max? Moreover, the first and second derivative test fail..etc