Cauchy inequality seems to be often used in math competitions, but it doesn't seem to need much time in the college entrance examination.

Polar coordinates are also common in competitions, which is convenient for understanding some topics.

Personally, I think I can buy a competition book to read. Cauchy inequality has a lot in the inequality chapter, which guarantees that you will be soft. But Cauchy inequality is more skillful, and many times you have to make some efforts according to the topic, so the difficulty is still there.