Official solution
Given : The vertices of a right angled triangle \[A\left( {1,k} \right),\]
\[B\left( {1,1} \right)\] and \[C\left( {2,1} \right)\] and area of \[\Delta ABC = 1\] square unit
We know that, area of night angled triangle
\[ = {1 \over 2} \times BC \times AB = 1 = {1 \over 2}\left( 1 \right)\left| {\left( {k - 1} \right)} \right|\]
\[ \Rightarrow \pm \left( {k - 1} \right) = 2 \Rightarrow k = - 1,3\]