Question

Find the area of whose vertices are$A(1,1,2)$ $B(2,3,5)$ and . $C(1,5,5)$

Answer 1

Ar $=\frac{1}{2}\left|\begin{array}{ccc}1& 1& 2\\ 2& 3& 5\\ 1& 5& 5\\ 1& 1& 2\end{array}\right|=\frac{|1(-5)-1(-8)+2\left(3\right)|}{2}=\frac{9}{2}$