1. The problem statement, all variables and given/known data 2. Relevant equations the equation is given in the question 3. The attempt at a solution i tried replacing the 2/3 pi rad as the cos argument and i obtained 2. but the mark scheme says that the position is 3cm above AB. in the next part, we have a phase diff of 4/3 pi and i again obtained 2 but the MS still says 3cm above AB. can someone help. am i doing a mistake here.