Art of problem solving has techniques, books, videos, etc. https://artofproblemsolving.com/resources https://artofproblemsolving.com/resources There's also htt

Art of problem solving has techniques, books, videos, etc. https://artofproblemsolving.com/resources https://artofproblemsolving.com/resources There's also http://brilliant.org http://brilliant.org which is useful