Exercise 1.6.9

We select different combinations of $a$ and $d$ to have $a+d=9$, then make the determinant of $A$ equal to 20.

• $A_1=\left[\begin{array}{cc}\hfill 4\hfill & \hfill 1\hfill \\ \hfill 0\hfill & \hfill 5\hfill \end{array}\right]$
• $A_2=\left[\begin{array}{cc}\hfill 5\hfill & \hfill 0\hfill \\ \hfill 1\hfill & \hfill 4\hfill \end{array}\right]$
• $A_3=\left[\begin{array}{cc}\hfill 3\hfill & \hfill -1\hfill \\ \hfill 2\hfill & \hfill 6\hfill \end{array}\right]$