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Algebra of Matrices — Maths STD 12 Science — Question

Maharashtra BoardEnglish MediumSTD 12 ScienceMathsAlgebra of Matrices5 Marks
Question
If $\text{A}=\begin{bmatrix}\cos\theta&\text{i}\sin\theta\\\text{i}\sin\theta&\cos\theta\end{bmatrix},$ then prove by principle of mathematical induction that $\text{A}^\text{n}=\begin{bmatrix}\cos\text{n}\theta&\text{i}\sin\text{n}\theta\\\text{i}\sin\text{n}\theta&\cos\text{n}\theta\end{bmatrix}$ for all $\text{n}\in\text{N}.$
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Answer

We shall prove the result by the principle of mathematical induction on n.
Step 1: If n = 1, by definition of integral power of a matrix, we have
$\text{A}^1=\begin{bmatrix}\cos1\theta&\text{i}\sin1\theta\\\text{i}\sin1\theta&\cos1\theta\end{bmatrix}=\begin{bmatrix}\cos\theta&\text{i}\sin\theta\\\text{i}\sin\theta&\cos\theta\end{bmatrix}=\text{A}$
Thus, the result is true for n = 1.
Step 2: Let the result be true for n = m. Then,
$\text{A}^\text{m}=\begin{bmatrix}\cos\text{m}\theta&\text{i}\sin\text{m}\theta\\\text{i}\sin\text{m}\theta&\cos\text{m}\theta\end{bmatrix}$
Now we shall show that the result is true for n = m + 1.
Here,
$ \text{A}^\text{m+1}=\begin{bmatrix}\cos\text{m}+1\theta&\text{i}\sin\text{m}+1\theta\\\text{i}\sin\text{m}+1\theta&\cos\text{m}+1\theta\end{bmatrix}$
By definition of integral power of matrix, we have
$\text{A}^\text{m+1}=\text{A}^\text{m}\text{A}$
$\Rightarrow\text{A}^\text{m+1}=\begin{bmatrix}\cos\text{m}\theta&\text{i}\sin\text{m}\theta\\\text{i}\sin\text{m}\theta&\cos\text{m}\theta\end{bmatrix}\begin{bmatrix}\cos\theta&\text{i}\sin\theta\\\text{i}\sin\theta&\cos\theta\end{bmatrix}$ [From eq. (1)]
$\Rightarrow\text{A}^\text{m+1}=\begin{bmatrix}\cos\text{m}\theta.\cos\theta+\text{i}\sin\text{m}\theta.\text{i}\sin\theta&\cos\text{m}\theta.\text{i}\sin\theta+\text{i}\sin\text{m}\theta.\cos\theta\\\text{i}\sin\text{m}\theta.\cos\theta+\cos\text{m}\theta.\text{i}\sin\theta&\text{i}\sin\text{m}\theta.\text{i}\sin\theta+\cos\text{m}\theta.\cos\theta\end{bmatrix}$
$ \Rightarrow\text{A}^{\text{m}+1}=\begin{bmatrix}\cos\text{m}\theta.\cos\theta-\sin\text{m}\theta.\sin\theta&\text{i}(\cos\text{m}\theta.\sin\theta+\sin\text{m}\theta.\cos\theta)\\\text{i}(\sin\text{m}\theta.\cos\theta+\cos\text{m}\theta.\sin\theta)&-\sin\text{m}\theta.\sin\theta+\cos\text{m}\theta.\cos\theta\end{bmatrix}$
$\Rightarrow\text{A}^\text{m+1}=\begin{bmatrix}\cos\text{m}\theta.\cos\theta-\sin\text{m}\theta.\sin\theta&\text{i}(\cos\text{m}\theta.\sin\theta+\sin\text{m}\theta.\cos\theta)\\\text{i}(\sin\text{m}\theta.\cos\theta+\cos\text{m}\theta.\sin\theta)&\cos\text{m}\theta.\cos\theta-\sin\text{m}\theta.\sin\theta\end{bmatrix}$
$\Rightarrow\text{A}^\text{m+1}=\begin{bmatrix}\cos(\text{m}\theta+\theta)&\text{i}\sin(\text{m}\theta+\theta)\\\text{i}\sin(\text{m}\theta+\theta)&\cos(\text{m}\theta+\theta)\end{bmatrix}$
$ \Rightarrow\text{A}^\text{m+1}=\begin{bmatrix}\cos(\text{m}+1)\theta&\text{i}\sin(\text{m}+1)\theta\\\text{i}\sin(\text{m}+1)\theta&\cos(\text{m}+1)\theta\end{bmatrix}$
This shows that when the result is true for n = m, it is true for n = m + 1.

Hence, by the principle of mathematical induction, the result is valid for all $\text{n}\in\text{N}.$

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