The one thing my wife (a math teacher) has always been craving is a video solution for these - that is, write equations or annotate a document on our android tablet while she narrates, then upload the video to YouTube. Last I checked there wasn't a good offering on the play store for this.

I do that with Screenflow and a Wacom Intuos Pro, and that's a very simple set up.

I understand it's not a solution for the Android tablet, but I also thought I wanted to draw on a tablet and I've been happy with my set up - I can sit up and look forward and it feels better that way, than it would if I looked down towards my drawing hand and the tablet screen.

Just a thought. I'm one of those people who would never buy a Wacom Cintiq because I see the decoupling of "object to draw on" and "object to look at while drawing" as an incredible advantage.

What app does Khan Academy use for precisely this?

Here is a collaborative file that people have affectionately named Bikini Bottom Dinosaur. Check it out for an example. :) http://thesavior.github.io/draw/#0B9XIM4D3BWcWOU44bzVmNWJSd1...

Cool, feels as fast as a native app on my iPad. I like the infinite zooming and panning.