Khan Academy
khanacademy.org
Explains school maths and science one short video and one exercise at a time
Free accountClassic
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desmos.com
Type an equation and watch the curve, with a slider for every parameter.
Type y = a sin(bx) and Desmos offers to make a and b sliders, so the relationship between a coefficient and a shape becomes something you can drag rather than imagine. It handles inequalities, parametrics, polars, regressions and tables, and the whole thing is fast enough to use while explaining something to someone. Graphs get a shareable link, which is why teachers use it as a whiteboard.
Questions
Works the moment the page loads. No account, no email, no trial. Saving and naming graphs needs a free account, and it plots and fits rather than doing symbolic algebra — it will not hand you an integral in closed form.
No. Desmos Graphing Calculator does the work inside your browser, so the file never leaves your device — you can load the page, disconnect from the network, and it still works. That makes it a reasonable choice for material containing personal data.
Saving and naming graphs needs a free account, and it plots and fits rather than doing symbolic algebra — it will not hand you an integral in closed form.
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khanacademy.org
Explains school maths and science one short video and one exercise at a time
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Free lessons and endless drills for notation, intervals, chords and ear training
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Peer-reviewed university textbooks you can read online or download as PDFs
wikipedia.org
Gets you oriented on almost any subject, with the sources listed at the bottom