

InterviewSolution
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If B, C are n rowed matrices and if A = B + C, BC = CB, C2 = O, then show that for every n ϵ N, An+1 = Bn(B + (n + 1)C). |
Answer» Given, A = B + C, BC = CB and C2 = O. We need to prove that, An+1 = Bn(B + (n + 1)C). We will prove this result using the principle of mathematical induction. Step 1: When n = 1, we have An+1 = A1+1 ⇒ An+1 = B1(B + (1 + 1)C) ∴ An+1 = B(B + 2C) For the given equation to be true for n = 1, An+1 must be equal to A2. It is given that, A = B + C And we know, A2 = A × A. ⇒ A2 = (B + C)(B + C) ⇒ A2 = B(B + C) + C(B + C) ⇒ A2 = B2 + BC + CB + C2 However, BC = CB and C2 = O. ⇒ A2 = B2 + CB + CB + O ⇒ A2 = B2 + 2CB ∴ A2 = B(B + 2C) Hence, An+1 = A2 and the equation is true for n = 1. Step 2 : Let us assume the equation true for some n = k, where k is a positive integer. ⇒ Ak+1 = Bk (B + (k + 1)C) To prove the given equation using mathematical induction, we have to show that, Ak+2 = Bk+1(B + (k + 2)C). We know, Ak+2 = Ak+1 × A. ⇒ Ak+2 = [Bk(B + (k + 1)C)](B + C) ⇒ Ak+2 = [Bk+1 + (k + 1)BkC)](B + C) ⇒ Ak+2 = Bk+1(B + C) + (k + 1)BkC(B + C) ⇒ Ak+2 = Bk+1(B + C) + (k + 1)BkCB + (k + 1)BkC2 However, BC = CB and C2 = O. ⇒ Ak+2 = Bk+1(B + C) + (k + 1)BkBC + (k + 1)BkO ⇒ Ak+2 = Bk+1(B + C) + (k + 1)Bk+1C + O ⇒ Ak+2 = Bk+1(B + C) + Bk+1[(k + 1)C] ⇒ Ak+2 = Bk+1[(B + C) + (k + 1)C] ⇒ Ak+2 = Bk+1[B + (1 + k + 1)C] ∴ Ak+2 = Bk+1[B + (k + 2)C] Hence, The equation is true for n = k + 1 under the assumption that it is true for n = k. Therefore, By the principle of mathematical induction, the equation is true for all positive integer values of n. Thus, An+1 = Bn(B + (n + 1)C) for every n ϵ N. |
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