How to Teach Yourself Math (2018)

Which topic are you trying to learn? The books and other resources people suggest will probably depend heavily on the specific topic.

AoPS Alcumus.

Beyond that, OCW is pretty good. Find homework assignments from good courses.

As a footnote, the linked-to article is mostly nonsense. If you want to learn math:

- Work on good problems

- Once you've struggled for a bit, go back and look up how to do them

Yes, this includes problems you don't have prerequisites for and have no chance of solving. You shouldn't start by watching videos or explainers until you've struggled. There's a pile of science behind this.

Khan Academy. It has courses for all levels of difficulty. It has both theory and exercises. The exercises are even gamified (you get all sorts of badges, and encouragements). Pick one level, and see how it feels. If it feels a bit hard, don't try to grind it out. Pick a lower level. It's better to find thing a bit on the easy side, this way you build up some momentum. Obviously, you don't need to start with addition and multiplication. Start with something that feels very easy, but still puts a smile on your face when you solve problem after problem.

3Blue1Brown is unparalleled in terms of making math fun. But it's just entertainment. It does not stick. It is fantastic for people who already have a degree in math, for them watching a 3Blue1Brown beats watching a Netlix movie every time. For other people too. But for both advanced and beginners, 3Blue1Brown does not stick.

Only practice makes thing stick. And practice you can do in two ways: either you need to (you're in school, and that's your homework, or you have a job and you have a deliverable that depends on you grokking the Fourier transform), or you find that practice is fun. It's very, very difficult to make math exercises fun. Math is not skiing, or ping-pong, or even piano. It's just hard. Practicing piano may not be pleasant until you get quite good at it, but practicing math is even less pleasant, and it takes longer to get good at it. But Khan Academy just makes this math practice as much fun as it can get.

By the way, I don't have any sort of relationship with Khan Academy. I just genuinely think they are great. It would not surprise me if one day Khan gets a Nobel prize. He's actually helping humanity a lot.