Пусть одно чило х, второе - 5х,
х+5х=19
6х=19
х=19/6
х=3 1/6
3 1/6*5=15 5/6
Искомые числа 3 целых 1/6 и 15 целых 5/6
Проверка
3 1/6 + 15 5/6=18 6/6=19
Cosπ/5cos2π/5=(sinπ/5cosπ/5cos2π/5)/sinπ/5=
=sin2π/5cos2π/5/(2sinπ/5)=sin4π/5/(4sinπ/5)=
=sin(π-π/5)/(4sinπ/5)=sinπ/5/(4sinπ/5)=1/4