High school students, studying physics competitions, want to master basic advanced mathematics knowledge, such as differential equations. Are there any good exercise books (no textbooks)?
Advanced Mathematics (liberal arts) edited by Liao Fei Tsinghua University Publishing House.
Because you are still a high school student, use something shallow, not too deep, just use liberal arts.
If you do some problems, you can use "Selected Solutions of Jimmy Dovich's Advanced Mathematics Exercises" to get a general understanding.