<ol>
<li><p>If the double-derivative is increasing both before and after the point. Or if the double-derivative is decreasing both before and after the point.</p></li>
<li><p>Pretty much. A relative max/min usually has f’(x)=0, but that’s not necessarily true for the global max/min.</p></li>
<li><p>Slope. Take (a,f(a)) and (b,f(b)) and plug it into y2-y1/x2-x1.</p></li>
<li><p>Yes. R^2-r^2.</p></li>
</ol>