[tex]A_{\triangle ABC}= \frac{1}{2} \cdot |\triangle|~,unde\\
\triangle= \left|\begin{array}{ccc}x_A&y_A&1\\x_B&y_B&1\\x_C&y_C&1\end{array}\right| \\
\triangle= \left|\begin{array}{ccc}4&2&1\\1&1&1\\-1&3&1\end{array}\right| =-8\\
A_{\triangle ABC}= \frac{1}{2} \cdot |-8|=4[/tex]