In: Advanced Math
Let A be a square matrix defined by \( A = \begin{pmatrix}3&2\\ 3&-2\end{pmatrix} \)
(a) Find the characteristic polynomial of A.
(b) Show that A is diagonalizable then diagonalize it.
(c) Write \( A^n \) in term of n.
Solution
(a) Find the characteristic polynomial of A.
\( \implies P(\lambda)=|A-\lambda I|=\lambda^2+\lambda-12=\bigg(\lambda-4\bigg)\bigg(\lambda+3\lambda\bigg) \)
Therefore, the characteristics polynomial is
\( P(\lambda)=|A-\lambda I|=\bigg(\lambda-4\bigg)\bigg(\lambda+3\lambda\bigg) \)
(b) Show that A is diagonalizable then diagonalize it.
since, \( P(\lambda)\hspace{2mm}splitted \implies A \) is diagonalizable.
\( \bullet\hspace{2mm} For \hspace{2mm}\lambda_1=4 \) \( \implies E_4=span\left\{\begin{pmatrix}2\\ 1\end{pmatrix}\right\} \)
\( \bullet\hspace{2mm} For\hspace{2mm}\lambda_2=-3 \) \( \implies E_{-3}=span\left\{\begin{pmatrix}-1\\ 3\end{pmatrix}\right\} \)
Therefore, \( A=PDP^{-1} \) where
\( P=\begin{pmatrix}2&-3\\ 1&3\end{pmatrix} ,D=\begin{pmatrix}4&0\\ 0&-3\end{pmatrix} ,P^{-1}=\begin{pmatrix}\frac{3}{7}&\frac{1}{7}\\ -\frac{1}{7}&\frac{2}{7}\end{pmatrix} \)
(c) Write \( A^n \) in term of n.
since. \( A=PDP^{-1} \implies A^n=\bigg(PDP^{-1}\bigg)^n=PD^nP^{-1} \)
\( \implies A^n=\begin{pmatrix}2&-3\\ 1&3\end{pmatrix}\begin{pmatrix}4^n&0\\ 0&\left(-3\right)^n\end{pmatrix}\begin{pmatrix}\frac{3}{7}&\frac{1}{7}\\ -\frac{1}{7}&\frac{2}{7}\end{pmatrix} \)
\( =\begin{pmatrix}\frac{3\cdot \:2^{2n+1}+3\left(-3\right)^n}{7}&\frac{2^{2n+1}-6\left(-3\right)^n}{7}\\ \frac{3\cdot \:4^n-3\left(-3\right)^n}{7}&\frac{4^n+6\left(-3\right)^n}{7}\end{pmatrix} \)
Therefore, \( A^n=\begin{pmatrix}\frac{3\cdot \:2^{2n+1}+3\left(-3\right)^n}{7}&\frac{2^{2n+1}-6\left(-3\right)^n}{7}\\ \frac{3\cdot \:4^n-3\left(-3\right)^n}{7}&\frac{4^n+6\left(-3\right)^n}{7}\end{pmatrix} \)
Answer
Therefore.
a). \( P(\lambda)=|A-\lambda I|=\bigg(\lambda-4\bigg)\bigg(\lambda+3\lambda\bigg) \)
b). \( A=PDP^{-1} \) where \( P=\begin{pmatrix}2&-3\\ 1&3\end{pmatrix} ,D=\begin{pmatrix}4&0\\ 0&-3\end{pmatrix} \),\( P^{-1}=\begin{pmatrix}\frac{3}{7}&\frac{1}{7}\\ -\frac{1}{7}&\frac{2}{7}\end{pmatrix} \)
c). \( A^n=\begin{pmatrix}\frac{3\cdot \:2^{2n+1}+3\left(-3\right)^n}{7}&\frac{2^{2n+1}-6\left(-3\right)^n}{7}\\ \frac{3\cdot \:4^n-3\left(-3\right)^n}{7}&\frac{4^n+6\left(-3\right)^n}{7}\end{pmatrix} \)