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I learn MathJax by looking at the source of a formula, whether it's here or in MathJax website. However, if the symbol I need cannot be found on the formulas I can find, then I'm stuck.
Any idea/ quick reference on MathJax?
MathJax uses LaTeX syntax, so if you know how to typeset LaTeX equations, you know most of MathJax. See also this answer.
You could try Detexify2 if there are symbols that you don't know the code (but note that not all symbols are supported in MathJax).