Did somebody say B = (X'X)^(-1)X'Y? Here is one of my favorite OLS-related abuses of linear algebra: y = XB + e ---> e = y - XB ---> e = y - X(X'X)^(-1)X'Y ---

Did somebody say B = (X'X)^(-1)X'Y? Here is one of my favorite OLS-related abuses of linear algebra: y = XB + e ---> e = y - XB ---> e = y - X(X'X)^(-1)X'Y ---> e = [I - X(X'X)^(-1)X'Y]y = My Now use the two idempotent matrices to compute the SSR: e'e = (My)'(My) ---> e'e = y'M'My ---> e'e = y'MMy ---> e'e = y'My