12. 比较下列各组数的大小：
(1) $ \sqrt[3]{1 1} $与2； (2) $ \frac{8}{3} $与 $ \sqrt[3]{2 0} $； (3）-4， $ -\sqrt{1 5},-\sqrt[3]{1 0 0}. $

12.解:(1)$\because11＞8$,
$\therefore\sqrt[3]{11}＞\sqrt[3]{8}$.
$\therefore\sqrt[3]{11}＞2$.
(2)$\because\dfrac{512}{27}＜20$,
$\therefore\sqrt[3]{\dfrac{512}{27}}＜\sqrt[3]{20}$.
$\therefore\dfrac{8}{3}＜\sqrt[3]{20}$.
(3)$\because16＞15$,
$\therefore\sqrt{16}＞\sqrt{15}$.
$\therefore4＞\sqrt{15}$.
$\therefore-4＜-\sqrt{15}$.
$\because64＜100$,
$\therefore\sqrt[3]{64}＜\sqrt[3]{100}$.
$\therefore4＜\sqrt[3]{100}$.
$\therefore-4＞-\sqrt[3]{100}$.
$\therefore-\sqrt[3]{100}＜-4＜-\sqrt{15}$.