What I known:
Are there any other website for learning math? Which one do you think is the best?
closed as not a real question by Pete L. Clark, Thomas, vonbrand, Paul, Davide Giraudo Mar 30 '13 at 9:36
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See also: MIT OpenCourseWare. The link will direct you to MIT's offerings in mathematics. There's a whole range of classes and topics that are listed: and the courses vary in terms of the material provided: lecture notes, syllabi, video lectures, etc. The site is free to use. They also have a scholar program (free of charge), with classes in which you can "enroll" (no tuition/no credit) which allow you to actively "participate" in classes. (See link for more details.)
In fact, you might want to check out Wikipedia's entry on Open Course Ware, where you'll find a handful of links to universities and programs, similar to MIT's in principle, with no costs involved or formal enrollment required. Click on the links of interest, and search for course offerings in mathematics.
I'd also suggest that to get the most out of Open Course Ware resources, that you buy/borrow the recommended/required text(s) that were/are used for the class, and study it along with the following and using the resources available at the OCR websites. That way you are not restricting yourself to "only" what is available on-line, and you'll have the resources you might need to tackle reading a text and doing exercises. There is really no way of replacing, entirely, the value of studying a textbook, be it an on-line text, or a "hard copy" text: reading it, taking notes, working alongside proofs to fill in gaps, and doing the exercises to gauge your understanding of the material.
See also this earlier post: On-line resources for learning mathematics. There are many suggestions available there.
There's a ton of websites for math:
You could use http://tutorial.math.lamar.edu/ as a reference for algebra, calculus and differential equations.
http://www.purplemath.com is another okay site for beginning algebra.
I've also found http://www.mathcs.org/ to be a good reference for real analysis.
It all just depends on what your preference is and what you're studying. But like the comments said, using textbooks is a way better (and usually more reliable) source for learning mathematics.
Here's an example of why website's are in general worse for studying: What does Khan Academy have to offer? Depth? Rigor? And look at Vipul's post. Obviously Vipul is just talking about sloppiness on Khan Academy but this definitely can translate to online sites that aren't as peer reviewed as textbooks are.
If you are learning math at school I thing you are kind of forced to use books and follow a schedule, but if you are learning it for fun like me, I think one good way to learn is to take a single unresolved problem of math (better if basic and simple to state), of a single area you like, as for example geometry, number theory, graph theory or else and try to solve it youself. When you get stuck, learn the math you miss and try to move a step ahead. Use blogs, and this site to ask for help, and have fun.
Search for this on the web: What are some mathematical questions that are fairly easy to state but very difficult to prove.