Three more tips:

1. Find and copy Unicode math symbols on Mathematical operators and symbols in Unicode - Wikipedia

2. Try http://embed.fun for a great-looking tool that makes it easy to embed equations in Medium.

2. You can do a lot with Unicode superscripts and subscripts, even for summations and integrals with limits, like

*∫ₐᵇ x*²

∑*ᵢ xᵢ*²

∑ *ᵢ*₌₀*ⁿ*⁻¹ *xᵢ*²

It doesn't look perfect but it's readable on the majority of computers (can depend on installed fonts) and doesn't use any images or embeds. It actually looks better in the body of a Medium article than here in the Responses section. I'm writing an article on how to do this and more without images or embeds.