Question

arccos(sin 4П/5)=????????????

Answer 1

$arccos (sinfrac{4pi}{5})=arccos(sin(pi -frac{pi}{5}))=arccos (sin(frac{pi }{5}))=\\=arccos(sin(underbrace {frac{pi }{2}-frac{3pi}{10}}_{pi /5}))=arccos(cosfrac{3pi}{10})=frac{3pi}{10}; ;\\frac{3pi}{10}in ; 1; chetverti$