Straight-line graphs · Year 9
In Year 9 you learn: gradient and y-intercept.
Every straight line can be written as y = mx + c. The gradient m says how steep the line is, and c says where it crosses the y-axis. Straight lines model anything that changes at a steady rate, such as a phone bill or a taxi fare.
Key facts
- Gradient = change in y ÷ change in x (“rise over run”).
- A positive gradient slopes up to the right; a negative gradient slopes down.
- Parallel lines have the same gradient.
Worked examples
Example 1
Find the gradient and y-intercept of 2y = 6x − 8.
Answer: Gradient 3, y-intercept −4
Example 2
Find the equation of the line through (1, 5) and (4, 11).
Answer: y = 2x + 3