Determine the exact values of the following expressions. Use reference angles where necessary.
Note: Your answers must be in EXACT form. They cannot contain decimals. If an answer involves a square root, write it as sqrt . For instance, the square root of 2 should be written as sqrt(2).

$\sin (-2\pi) =$
$\cos (\frac{\pi}{3}) =$
$\sin (-\frac{5\pi}{2}) =$
$\cos (-\frac{5\pi}{6}) =$
$\sin (-\frac{13\pi}{4}) =$
$\cos (\frac{3\pi}{4}) =$