Sejam $X=\left(\begin{array}{cc}{x}_{11}& x_{12}\\ x_{21}& x_{22}\end{array}\right)$ e $Y=\left(\begin{array}{cc}{y}_{11}& y_{12}\\ y_{21}& y_{22}\end{array}\right)$ matrizes quadradas $2\times 2$.

Definimos as matrizes: $\alpha \cdot X,X+Y$ e $X\cdot Y$ ($\alpha$=número real) por:$$\alpha \cdot X=\left(\begin{array}{cc}\alpha x_{11}& \alpha x_{12}\\ \alpha x_{21}& \alpha x_{22}\end{array}\right)\text{ e }X+Y=\left(\begin{array}{cc}{x}_{11}+y_{11}& x_{12}+y_{12}\\ x_{21}+y_{21}& x_{22}+y_{22}\end{array}\right)$$$$X\cdot Y=\left(\begin{array}{cc}{x}_{11}{y}_{11}+x_{12}{y}_{21}& x_{11}{y}_{12}+x_{12}{y}_{22}\\ x_{21}{y}_{11}+x_{22}{y}_{21}& x_{21}{y}_{12}+x_{22}{y}_{22}\end{array}\right)$$Uma das afirmações abaixo e verdadeira, assinale-a: