Perhaps this thread can give you the answer to your first question (it worked for me):

http://moodle.org/mod/forum/discuss.php?d=55861

2. The 'array' syntax should work ok then:

$$$\begin{array}{rcl} f(x) & = & \cos{x}\\ f'(x) & = & - \sin x\\ \int_{0}^{x} f({y}) dy & = & \sin{x} \end{array}$$$

$$\begin{array}{rcl} f(x) & = & \cos{x}\\ f'(x) & = & - \sin x\\ \int_{0}^{x} f({y}) dy & = & \sin{x} \end{array}$$

Best regards

Morten