1. The problem statement, all variables and given/known data 2. Relevant equations In question. 3. The attempt at a solution To be clear it's part (vi) that's unclear to me. In order to ignore the cosine term it has to reduce to 1. This can happen, only if k(x1+x2)/2 = ωt Is this a correct assumption ? Also, it is known that k = 2∏/λ and ω=2∏/T However, I'm trying to think in what way these two components could be equal but I can't get it. Could someone give me some help, please ?