Please answer the above question.Please show all calculations.Please show which formula's you have used to answer the question.Please include explanations where possible.Thank you.Much appreciated.

1.2 Let A be an mm matrix partitioned as A=[A11A21A12A22], where A11 is m1m1 and rank(A)=rank(A11)=m1. 1.2.1 Show that A22=A21A111A12. 1.2.2 Use the result of part 1.2.1 to show that B=[A111(0)(0)(0)] is a generalised inverse of A