Perpendicular Lines

1. Block 1
Perpendicular Lines

2. What is to be learned?
• The rule connecting gradients and
perpendicular lines.

3. x
y
(2, 2)
(7 , 6)
m = 6 – 2
7 – 2
= 4
/5
(-2 , 7)
m = 7 – 2
-2 – 2
= - 5
/4
m1
m2

4. x
y
(1, 3)
(8 , 5)
m = 5 – 3
8 – 1
= 2
/7
(-1 , 10)
m = 10 – 3
-1 – 1
= - 7
/2
m1
m2

5. Summarising
m1 m2
4
/5
2
/7
-9
/11
4
-5
/4
-7
/2
11
/9
1 - ¼
4
/5 X -5
/4 = - 20
/20
= -1
m1m2 = -1

6. For perpendicular lines
m1m2 = -1
If AB is ppdlr to CD
Ex 1 mAB = 6
/7
Ex 2 mAB = - 3
/8
Ex 3 mAB = 5
mAB = 0!!!
mCD = - 7
/6
mCD = 8
/3
(5
/1) mCD = - 1
/5
mCD = ∞

7. Parallel and Perpendicular
Which lines are perpendicular
Which lines are parallel?
y = 4x + 7 y = ½x +2
y = ¼x y = – 5 + 4x
y = -2x + 4 y = 2x – 4