You can also buy a classified compilation of senior high school entrance examination questions, which are real questions classified according to various knowledge points (all in bookstores), and one of them is called comprehensive questions, which is the finale.
The finale is generally based on basic knowledge, but it is comprehensive and impossible. In the final analysis, the foundation is not solid and the application ability is not strong. Decompression is the key.
So just prove that they can't be equal.
Suppose the element {a, b, c, ...} is in x and the elem