A+ Click is an interactive collection of more than 3700 math problems and answers for K-1 K-12 school program. It defines the personal level of math knowledge. You move up into the next level if you give 5 correct answers in a row. Practice makes perfect.
This is a free textbook which covers material for an introductory course on differential equations with some partial differential equations material, though it assumes knowledge of matrix theory. It includes a section on computing Fourier series of polynomials. It also includes a link to the freely available student solutions manual.