1.

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 = Aand 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)Bk

⇒ 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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