Respuesta :

evgeniylevi
Try this option:

1. rule: cosα=sin(90°-α), it means that cos 3x=sin(90°-3x);
2. sin(90°-3x)=sin(x+18°); ⇔ 90°-3x=18°+x; ⇔ x=18°.

Answer: x=18°.