[tex]\bf \begin{cases}
a=5i+7j\implies &\ \textless \ 5,7\ \textgreater \ \\
b=-4i+3j\implies &\ \textless \ -4,3\ \textgreater \
\end{cases}
\\\\\\
\ \textless \ 5,7\ \textgreater \ \cdot \ \textless \ -4,3\ \textgreater \ \implies (5\cdot -4)+(7\cdot 3)\implies -20+21\implies 1[/tex]
