Offering a powerful yet easy-to-understand approach, this book presents graduate and upper-level undergraduate students with an active learning experience. The focus is placed on helping learners grasp central mathematical themes rather than peripheral details, reducing much of the time-consuming task of memorizing formulae. Numerous examples and interactive "Problems for the Reader" are generously interspersed throughout to help learners absorb concepts more effectively. Exercises in each chapter are carefully designed to strengthen students' ability to solve general problems. Throughout the learning process, the author also shares valuable problem-solving tools and tips acquired through more than 40 years of research and teaching.