Parallel and perpendicular lines
In a nutshell
Lines can be parallel or perpendicular to each other. You can find out by looking at the gradients of the lines. If two lines are parallel, they share the same gradient. Two perpendicular lines will have the product of their gradients equal to −1. If you have the gradient of one line (m), the gradient of a perpendicular line is equal to −m1.
Parallel lines
Two lines that are parallel will have the same gradient. This is apparent if you look at a graph with parallel lines. They share the same value of m.
Example 1
Prove that the lines l1 (y=3x+2) and l2 (6x−2y−6=0) are parallel.
Rearrange equation to give y=mx+c . See if m of l1 is equal to m of l2.
l1 ml2 is 6x−2y−6−2y2yyl2 m=3=0=−6x+6=6x−6=3x−3=3
l1=l2=3, therefore the lines are parallel.
Perpendicular lines
For perpendicular lines, the slope of one line is the negative reciprocal of its perpendicular counterpart. Take the gradient of one line and find the negative reciprocal (−m1) to find the gradient of the perpendicular line. The product of the gradients of the two perpendicular lines will equal −1, m1×m2=−1.
Example 2
Investigate if the lines y=4x+2 and 2x+8y−12=0 are perpendicular.
Rearrange equation to give y=mx+c.
2x+8y−128yyym=0=−2x+12=−82x+812=−41x+23=−41
Multiply the gradients to get −1.
m1=4,m2=−41
4×−41=−1
m1×m2=−1, therefore the lines are perpendicular.