write each of the following as the ratios of positive acute angles.

in the examples i understand that $\displaystyle sin 330 = -sin 30$

but can someone please explain to me how it is that

$\displaystyle cos (3/4) \pi $= $\displaystyle -cos (1/4) \pi$

and $\displaystyle sin -(4/5) \pi $= $\displaystyle -sin (1/5) \pi$

i understand that $\displaystyle \pi$ = 180 but i am not geting it when i express each of the sectors in terms of $\displaystyle \pi$