Find the adjoint of the matrix and hence show that A(adj A) = |A| I 3 .

Question

Philoid · Accepted Answer

A =

|A| =

= – 1(1 – 4) + 2(2 + 4) – 2( – 4 – 2)

= 3 + 12 + 12

|A| = 27

Cofactors of A

C₁₁ = – 3 C₂₁ = – 6 C₃₁ = 6

C₁₂ = – 6 C₂₂ = 3 C₃₂ = – 6

C₁₃ = – 6 C₂₃ = – 6 C₃₃ = 3

adj A =

=

So, adj A =

A(adj A) =

=

A(adj A) = 27

Hence, A(adj A) = |A|I