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Se consideră matricea [tex]$A(x)=\left(\begin{array}{cc}2^{x} & 0 \\ 0 & 3^{x}\end{array}\right)$[/tex], unde [tex]$x$[/tex] este număr real.

[tex]$5 \mathbf{p}$[/tex] a) Arătați că [tex]$\operatorname{det}(A(x))=6^{x}$[/tex], pentru orice număr real [tex]$x$[/tex]

5p b) Determinați numărul real [tex]$x$[/tex], știind că [tex]$A(x) \cdot\left(\begin{array}{ll}1 & 1 \\ 0 & 1\end{array}\right)=\left(\begin{array}{ll}1 & 1 \\ 0 & 1\end{array}\right) \cdot A(x)$[/tex].

5 ( c) Demonstraţi că, orice matrice [tex]$X \in \mathcal{M}_{2}(\mathbb{R})$[/tex] cu proprietatea că [tex]$X \cdot X=A(1)$[/tex] are două elemente numere iraționale.


Răspuns :

[tex]A(x)=\left(\begin{array}{cc}2^{x} & 0 \\ 0 & 3^{x}\end{array}\right)[/tex]

a)

Calculam det(A(x)), facem diferenta dintre produsul diagonalelor

det(A(x))=2ˣ×3ˣ-0=6ˣ

b)

[tex]\left(\begin{array}{cc}2^{x} & 0 \\ 0 & 3^{x}\end{array}\right)\cdot \left(\begin{array}{cc}1& 1 \\ 0 &1 \end{array}\right)=\left(\begin{array}{cc}1& 1 \\ 0 &1 \end{array}\right)\cdot \left(\begin{array}{cc}2^{x} & 0 \\ 0 & 3^{x}\end{array}\right)\\\\\left(\begin{array}{cc}2^x&2^x \\ 0 &3^x\end{array}\right)=\left(\begin{array}{cc}2^x& 3^x \\ 0 &3^x\end{array}\right)\\\\2^x=3^x\\\\x=0\\\\[/tex]

c)

[tex]Fie\ X=\left(\begin{array}{cc}a& b \\ c & d\end{array}\right)\\\\X\cdot X=\left(\begin{array}{cc}a& b \\ c & d\end{array}\right)\cdot \left(\begin{array}{cc}a& b \\ c & d\end{array}\right)=\left(\begin{array}{cc}a^2+bc& b(a+d) \\ c(a+d) & bc+d^2\end{array}\right)\\\\[/tex]

[tex]\left(\begin{array}{cc}a^2+bc& b(a+d) \\ c(a+d) & bc+d^2\end{array}\right)=\left(\begin{array}{cc}2& 0 \\ 0 &3\end{array}\right)[/tex]

a²+bc=2

b(a+d)=0⇒ b=0

c(a+d)=0⇒c=0

bc+d²=3

0+d²=3

d²=3

d=±√3∉R

a²=2

a=±√2∉R

Deci a si d sunt numere irationale

Un alt exercitiu cu matrice gasesti aici: https://brainly.ro/tema/9928531

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