To draw a line, we need two points on the line. The easiest two points to fi nd are the points where the line crosses the axes. These are known as the intercepts.
Graphing a line using two points on the line Find the y -intercept by letting x = 0 and fi nding y .
1
2 3
4
Find the x -intercept by letting y = 0 and fi nding x . Plot these two points. Draw a line which passes through these two points.
Worked example 2 Draw a graph of the line 3x − 2y − 12 = 0
Solution –1
–1 –2
–3 –4 –5 –6 –7
y
1 0
3x – 2y – 12 = 0 1 2 3 4
x-intercept (4, 0) x
5 6 7
y
3 2 1
–1 0 –1
y-intercept (0, 2)
x-intercept (5, 0)
1 2 3 4 5 6 x
On the x -axis, y = 0 On the y -axis, x = 0
y-intercept (0, –6)
Two interesting cases 1. Lines through the origin
If the constant in the equation is equal to zero, so that the equation is in the form ax + by = 0 , then the line passes through the origin (0, 0), and so the intercept method will only give us one point to plot. In this case, we must fi nd a diff erent point which is on the line.
Graphing a line through the origin 1
2 3
y (x, y)
Let x equal the number in front of the y (or any suitable number). Solve for y . Plot this point.
Draw a line that passes through the origin and this point. Section C Applying our skills 503