These are just lines that have the same gradient. Above you will see the lines for y = 1.5x +1 and y = 1.5x – 0.5.
It doesn’t matter what the value of c is, so long as as the m values are equal when expressed in the standard y = mx + c form then the lines are parallel.
Based on this you might be asked a question like: Line T has a gradient of 2 and is parallel to Line U. Line U passes through point (4,5). Find its equation.
As you know they are both parallel you know m = 2 and it goes through (4,5) so just plug these into the standard straight line graph equation to get c:
y = mx + c
5 = (2 x 4) + c = 8 + c
c = 5 – 8 = -3
Therefore the equation of the line is y = 2x -3.
These are lines that cross each other at right angles. There are two important rules to learn about such graphs:
- If the gradient of one line is m, then the gradient of the other is -1/m
- If you multiply both gradients together you get -1
The examples above show the lines y = 2.5x and y = -(1/2.5)x = -0.4x
Example: Find the perpendicular line to 4y – 3x = 8 through the point (0,2).
Rearrange the first equation into the form y = mx + c
So y = 3/4x + 2
Find the value of the perpendicular line’s gradient.
If the first one is 3/4 then:
3/4 x m = -1
Therefore m = -1 ÷ 3/4 = -4/3
Put this, and the coordinates, into the standard format equation to find c.
y = mx + c
so 2 = (-4/3 x 0) + c
Therefore c = 2.
You now have c and m.
So the equation is y = -4/3x + 2.