The Math Reference Project
Science, Math, Reference - The Math Reference Project. An electronic archive of articles by Karl Dahlke on topics ranging from high school geometry up to graduate level topology.
http://www.mathreference.com/
Summation formulas
Science, Math, Reference - Summation Formulae. For polynomials of degrees up to 50, generated using the Euler-Maclaurin method.
https://polysum.tripod.com/
MATH-ASSISTANCE
Science, Math, Reference - Math Assistance. Lists rules and formulas for a number of mathematical subjects, such as plotting graphics, functions, factoring, derivatives, integrals, matrices, vectors, and numerical analysis.
http://www.ercangurvit.com/
Dave's Math Tables
Science, Math, Reference, Tables - Dave's Math Tables. Has math tables for many areas of math from general math to advanced calculus.
http://math2.org/
PlanetMath
Science, Math, Reference - PlanetMath. A collaborative encyclopaedia with entries contributed under the GNU Free Documentation License.
https://planetmath.org/
Basic Mathematics Skills
Do not spend expensive money on courses and softwares. My website is designed to give you a solid understanding of basic mathematics.
https://www.basic-mathematics.com/