1. The problem statement, all variables and given/known data I could not type it here, so i made a screenshot and posted it below. 2. Relevant equations it's about complex numbers undergraduate level. i'm currently doing Euler's formula and De Moivre's theorem, although i'm not sure that the solution lies there. 3. The attempt at a solution i tried doing it by adding the values inside the cos() but when verifying via calculator with a dummy data, it didn't add up.