IGCSE Maths / Graphs of straight lines
Perpendicular lines revision notes
Edexcel International GCSE Maths (4MA1), chapter 6: Graphs of straight lines, specification point 3.3g. Part of the full course.
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Perpendicular lines cross at a right angle, and their gradients multiply to −1. To find a perpendicular line’s equation, find its gradient, use the given point to work out c, then check the gradients.