Помогите решить срочно. зарание спасибо.

Решите задачу:

$15(cos(2 \pi /3)+i*sin(2 \pi /3))=15(-1/2+i* \sqrt{3}/2)=\\=-7,5+7,5 \sqrt{3}*i \\\\ \frac{1}{cos(-3 \pi /4)+i*sin(-3 \pi /4)}= \frac{1}{- \sqrt{2}/2+ \sqrt{2}/2*i }=\\\\= \frac{- \sqrt{2}/2- \sqrt{2}/2*i }{(- \sqrt{2}/2+ \sqrt{2}/2*i )(- \sqrt{2}/2- \sqrt{2}/2*i )}= \frac{- \sqrt{2}/2- \sqrt{2}/2*i }{(- \sqrt{2}/2)^2- (\sqrt{2}/2*i)^2 }= \\\\= \frac{- \sqrt{2}/2- \sqrt{2}/2*i }{1/2+1/2}= \frac{- \sqrt{2}/2- \sqrt{2}/2*i }{1}=- \sqrt{2}/2- \sqrt{2}/2*i$