1. The problem statement, all variables and given/known data see the pictures 2. Relevant equations 3. The attempt at a solution one. In the third picture. 3rd line and 4th line. i don't understand why 1-e^(-t^2/n^2) become (1+t^2)/n^2 two. I'm not familiar with proving equality by showing difference going to zero. for example, I prove equality by showing that A equal B, B equal C, so A equal C. What is a good book to learn this way of proof ?