1. The problem statement, all variables and given/known data I am not sure how to get the Mmin . Why δA will become B (refer to the pink circled part) 2. Relevant equations 3. The attempt at a solution Is there anything wrong with the author's steps ? I know the author's intention is to show that the Mmin occur when the Fr = (Q^2)B/ g(A^3) = 1 , but we have (Q^2)δA / g(A^3) ... How to make the B=δA ???