Xi Xi is actually very smart and has a high IQ (that's what she said herself), but she just has a bad taste in advanced mathematics. It is said that there are historical reasons...
It seems that Xu Yang was not named Xu Yang originally. It is said that there is a number between the names, so learning advanced mathematics is like a duck in water ("because I found an organization" someone said). It is also said that he changed his name after someone accidentally made fun of him...
As a result, a "bad relationship" (also someone's word) that started in high-level mathematics class began to take place...