Consider the matrices A= bmatrix2 & -2\ 4 & -2 bmatrix and B= bmatrix3 & 9\ 1 & 3 bmatrix. If matrices P and Q are such that PA=B and AQ=B, then the absolute value of the sum of the diagonal elements of 2(P+Q) is .
Answer
34
Solution
|A|=-4+8=4 0, A-1= 14 bmatrix-2&2\\-4&2 bmatrix
PA=B P=BA-1= 14 bmatrix3&9\\1&3 bmatrix bmatrix-2&2\\-4&2 bmatrix= 14 bmatrix-42&24\\-14&8 bmatrix
tr(P)= -42+84=- 172
AQ=B Q=A-1B= 14 bmatrix-2&2\\-4&2 bmatrix bmatrix3&9\\1&3 bmatrix= 14 bmatrix-4&-12\\-10&-30 bmatrix
tr(Q)= -4-304=- 172 (as expected, tr(BA-1)=tr(A-1B))
tr (2(P+Q) )=2 (- 172- 172 )=-34
Absolute value =34
Answer: 34