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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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