Essence of Linear Algebra: Revised Span Lesson

[kurtbruns]

This pull request publishes these changes for the "span" lesson:

• Publishes vectorized images consistent with the rest of the series
• Adds 3d visuals with questions and animations, replacing the existing interactives
• Changes author's perspective to plural
• Removes a section about inconsistent notation from the article

[3b1b]

Great, this is an improvement. I noticed that the previous interactives were simply not working on my iPad with Safari.

The only thing that caught my eye as worth changing is that for the question "What is the span of...", the target audience may not have seen the notation $\mathds{R}^2$ before. It also doesn't quite make sense to compare it with R and 0, since in all cases the span is a subset of R^2, i.e. a set of pairs of numbers, not a set of numbers or an individual number. The goal of the question could be better accomplished just with the options A) A point, B) A line C) The full 2d plane.