Question

In Lec 7 Prof. Strang explains the how to find the Null space using -F,I.
the problem is the dimensions doesn't allways fit like in the last example he did when he just took part of I. can someone clarify this subject for me? how do use this method?

Answer 1

The I in the reduced echelon form is rxr. The I in the special solutions matrix (null space matrix) is (n-r)x(n-r). F is r x (n-r).