回复：继续做功课，用初等数学证明了做空FAZ/FAS赚钱的道理

What you have described is known. And a lot of people have been trying to game this fact. It can be profitable, but it requires a great deal of sophistication. Try watching these ETFs for a while (including minute by minute XLF correlation), do some paper trading, talk to traders, and then you might, just might, want to look deeper. I know it is beyond me for sure.