If you are looking for these resources, there are three primary places to look:
His famous opening line in the 18.06 lectures is: “The fundamental problem of linear algebra is to solve a system of linear equations.” But he doesn't stop there. He immediately introduces the —the idea that solving ( Ax = b ) is about finding the right combination of the columns of ( A ).
Given a matrix (A) with independent columns, the projection of (b) onto (C(A)) is: [ p = A(A^TA)^-1A^T b ] The projection matrix: (P = A(A^TA)^-1A^T). Properties: (P^T = P) and (P^2 = P).