Se consideră [tex]A(x)=\left([tex]\begin{array}{ccc}1 & x & 2 x^{2}+x \\ 0 & 1 & 4 x \\ 0 & 0 & 1\end{array}\right), x \in \mathbb{R}[/tex]. Arătați că:[/tex]

a) [tex]A(x) A(y)=A(x+y), x, y \in \mathbb{R}[/tex];

b) [tex]A\left(x_{1}\right) A\left(x_{2}\right) \ldots A\left(x_{n}\right)=A\left(x_{1}+x_{2}+\ldots+x_{n}\right), x_{k} \in \mathbb{R}, k=\overline{1, n}, n \in \mathbb{N} *[/tex]