This key facts covers The Gradient Rules within Parallel & Perpendicular for GCSE Mathematics. Revise Parallel & Perpendicular in Graphs for GCSE Mathematics with 14 exam-style questions and 12 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 5 of 10 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
The Gradient Rules
For Parallel Lines
If L₁ has gradient m₁ and L₂ has gradient m₂, then:
L₁ || L₂ ⟺ m₁ = m₂
For Perpendicular Lines
If L₁ has gradient m₁ and L₂ has gradient m₂, then:
L₁ ⊥ L₂ ⟺ m₁ × m₂ = -1
This means: m₂ = -1/m₁
Practice questions for Parallel & Perpendicular
Which of the following is true about two parallel straight lines?
Explain why a horizontal line and a vertical line are always perpendicular to each other. Your explanation must refer to gradients.