1. The problem statement, all variables and given/known data Hello all ! Over the past few day's, I've been trying to understand how Sean Carroll comes to the conclusion that he does on equation 1.153. I've tried to look for various resources online but I still have trouble understanding how he is able to add both partial derivatives in 1.153. 2. Relevant equations 3. The attempt at a solution I tried to replace the dummy indices with our initial indices on 1.151 yet that seems to not bring me to the conclusion that one is supposed to get on 1.153. I would appreciate any guidance or help thank you !