CPM Homework Help : PC Problem 12-51

Earlier we found the matrix associated with the composition of rotating $90°$ counterclockwise and then reflecting across the $x$ -axis. Now find the matrix associated with reflecting across the $x$ -axis and then rotating $90°$ .

Hint:

From problem 12-45:
$A =$ matrix associated with $90º$ counterclockwise rotation
$B =$ martix associated with reflecting across the $x$ -axis

$A = \left[ \begin{array} { c c } { 0 } & { 1 } \\ { - 1 } & { 0 } \end{array} \right] \quad B = \left[ \begin{array} { c c } { 1 } & { 0 } \\ { 0 } & { - 1 } \end{array} \right]$

Step:

Multiply $A$ and $B$ in the opposite order you did in problem 12-45. That is, find $BA$ .