1. The problem statement, all variables and given/known data Looking at the picture I have attached the problem is very straight forward. In the denominator of the equations I have posted the teacher uses 2[itex]\Pi[/itex] but the book uses 4[itex]\Pi[/itex]. Can someone please explain to me why is that. 2. Relevant equations Book's equation: Teacher's Soln: 3. The attempt at a solution Please refer to attached pictures.