When I learned mathematics at school and at college, various concepts and techniques were taught. The most powerful, and perhaps the most useful, was calculus. We learned a little about symmetry, but it was not regarded as powerful or useful; discussions of symmetry were more like "look at the pretty flower". I only realised much later that symmetry is a powerful tool for solving mathematical problems. Here are some that can be solved using symmetry.
This is one of several miscellaneous pages.