As soon as we require paid resources in the main curriculum, we might as well tell people to pay half a million dollars to attend a university.

We are an Internet-based community of learners, not a business, so free is the most sensible price and ensures that the only price you need pay is the price of Internet access.

Therefore, we respect the business model of websites like edX, which make their materials free but with some paid add-ons, like official certificates or extra interaction with course instructors.

So we only require that the *learning materials* of a resource be free to access, not that every possible add-on be free.

Without these assumptions, the curriculum would be out of control with trying to fill in your knowledge gaps.

But those who for whatever reason didn't get all the way through high school math and physics are in luck: you can find the content you need on [Khan Academy](https://www.khanacademy.org/).