Show that the area of the triangle with vertices $\mathbf{R}\left(x_1, y_1\right), \mathbf{S}\left(x_2, y_2\right)$, and $\mathbf{T}\left(x_3, y_3\right)$ is
$$
\frac{1}{2}\left|x_1\left(y_2-y_3\right)+x_2\left(y_3-y_1\right)+x_3\left(y_1-y_2\right)\right|
$$