1.

If A, B are square matrices of the same order, then prove that adj (AB) = (adj B) (adj A).

Answer»

We know that,

(AB) adj (AB) = |AB|I = adj (AB)(AB) …(i)

⇒ (AB) (adj B . adj A) = A . B adj B . adj A = A(B adj B) adj A

= A(|B|I) adj A  [∵ B adj B = |B|I]

= |B|(A . adj A)

= |B||A|I  [∵ A adj A = |A|I]

= |A||B|I

= |AB|I …(ii)

From (i) and (ii), we get

(AB) (adj AB) = AB (adj B . adj A)

Pre-multiplying both sides by (AB)–1, we get

(AB)–1 ((AB) adj AB) = (AB)–1 ((AB) adj B . adj A)

⇒ adj AB = adj B . adj A



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