If a function is convex, the maximum value on a closed interval occurs at the endpoints, while the minimum often occurs in the middle. Lee Peng Yee often demonstrates that if variables are not equal, one can adjust them to make them closer together, often increasing or decreasing the target sum, thereby finding the extremum.
You can find "Pure Mathematics" by Lee Peng Yee in various online platforms, including online bookstores and academic databases. Unfortunately, I couldn't find a direct PDF link for the book. However, you can try searching for online versions or purchasing a physical copy from a reputable bookstore or online retailer. pure maths lee peng yee pdf link