Straight-line graphs · Grade 11

In Grade 11 you learn: parallel and perpendicular lines.

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

• In y = mx + c, m is the gradient and c is the y-intercept.
• 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.
• Perpendicular gradients multiply to −1: if one is m, the other is −1/m.
• For the line through two points: find m, then substitute one point into y = mx + c to find c.

Worked examples

Other years

• Straight-line graphs · Grade 9
• Straight-line graphs · Grade 10