1. The problem statement, all variables and given/known data Basically the issue is Landau & Lifgarbagez mechanics says δl = [r2 + (l + r)2 - 2r(l + r)cosθ]1/2 - l ≈ r(l + r)θ2/2l 2. Relevant equations θ much less than 1 3. The attempt at a solution I've no idea how to get the thing on the far right. I'm assuming it's Taylor expansion or something like that but that's not something any of my classes have really explained. Also this isn't homework, I'm on summer holidays.