Posted by
janine on
.
The image of the point (1, 2) after a rotation of 180° about the origin is

math 
Damon,
(1,+2) by inspection
using 90 deg matrix
R90 =
+0 1
+1 +0
R180 is R90 * R90
+0 1+0 1
+1 +0+1 +0
which is
1 +0
+0 1

continued 
Damon,
1 +0 +1
+0 1 2
= (1, +2 )