Feedback on chap-geometry

[danmargalit]

In reference to version 224734d7be7ff0c4a8adb3a206356ea122112f33

I think that everything in this section is in service of three main facts, which convert algebra to geometry.

1. Ax=b is consistent iff b is in the span of the columns of A
2. Solutions to Ax=b and Ax=0 are parallel
3. Rank nullity.

This could be explained in the opener to Chapter 2. I have edited my slides to reflect this.