8.44 Chapter 8
Suppose we reflect a point Q(x , y) in a vertical line L to a new point
Ql, where the distance between Q and L is b units. There are two cases.
(i)
Q(x,y) Q'(x + 2b,y)
L
b units b units
After Q is reflected to the
right in L, the coordinates of
Ql are (x + 2b , y).
(ii)
Q(x,y)
Q'(x - 2b,y)
L
b units b units
After Q is reflected to the
left in L, the coordinates of
Ql are (x - 2b , y).
Note: (a) Since QQl is a horizontal line, the y-coordinates of Q and Ql
are the same.
(b) Distance between Q and Ql = 2b units
In the figure, l1 is a line parallel to the x-axis and l2 is
a line parallel to the y-axis. Q is the image when a point
P(-3 , 1) is reflected in l1.
(a) Find the coordinates of Q.
(b) R is the image when Q is reflected in l2. Find the
coordinates of R.
(a) Distance between P(-3 , 1) and l1
= [1 - (-2)] units
= 3 units
Diagram Clue
P(-3,1)
Q
y
x
m1
O
M(1,- 2)
3 units
3 units
` Coordinates of Q = (-3 , 1 - 2 # 3)
= (,)35
--
Level 2
Example 16
P(-3,1)
M(1 ,- 2)
y
x
m2
m1
O
Solution
The y-coordinate of
any point on the horizontal line
l1 is -2.
◀ reflected downwards
◀
[ Objective: Question related to reflection in a horizontal or vertical
line in a rectangular coordinate plane.]
Supp Example 16
[on P.T8.76g]