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codecogs
Unfortunately, as of Feb 25, 2018, steemit does not seem to support math mode. However, it is still possible to insert some math symbols using codecogs website https://www.codecogs.com/latex/eqneditor.php
For example, typing in \int_0^\infty f(x)\,dx = \frac{\pi}{2} in the yellow box will produce a corresponding math expression
which can be inserted to a steemit post by copying and pasting HTML code in the bottom.