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Suppose A and B be two ono-singular matrices such that `AB= BA^(m), B^(n) = I and A^(p) = I `, where `I` is an identity matrix. If `m = 2 and n = 5 ` then p equals toA. 30B. 31C. 33D. 81 |
Answer» Correct Answer - B `because AB = BA^(m) ` ` rArr B = A^(-1) BA^(m)` `therefore B^(n) = underset("n times")(underbrace((A^(-1) BA^(m))(A^(-1)BA^(m))... (A^(-1) BA^(m))))` `=A^(-1) underset("n times")(underbrace(BA^(m-1)BA^(m-1)... BA^(m-1)BA^(m-1)))A` ...(i) Given, ` AB = BA^(m)` ` rArr A AB = ABA^(m) = BA^(2m) rArr A A AB = BA^(3m)` Similarly, `A^(x) B = BA^(mx) AA m in N` From Eq. (i) we get `B^(n)=A^(-1) BA^(m-1) underset("(n-1) times")(underbrace(BA^(m-1)BA^(m-1)... BA^(m-1)BA^(m-1)))A` `=A^(-1) B(A^(m-1) B)A^(m-1) underset("(n-2) times")(underbrace(BA^(m-1)... BA^(m-1)BA^(m-1)))A` `=A^(-1) BBA^((m-1)m) A^(m-1) underset("(n-2) times")(underbrace(BA^(m-1)... BA^(m-1)BA^(m-1)))A` `=A^(-1) B^(2)A^((m^(2)-1)) underset("(n-2) times")(underbrace(BA^(m-1)... BA^(m-1)BA^(m-1)))A` `..." " ..." "... " "... ` `=A^(-1) B^(m) (A) ^(m^(n)-1)A` `I = A^(-1) I A^(m^(n)-1) A [because B^(n) = I]` `I = A^(-1) A^(m^(n)-1) A= A^(-1) A^(m^(n))` `rArr I = A^(m^(n)-1)` `therefore p= m^(n)-1 " "...(ii) [because A^(p) = I]` Put `m = 2, n = 5 ` in Eq. (ii), we get `p= 2^(5) - 1 = 31` |
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