Straight-line graphs
Lesson 13 of 13
Parallel and perpendicular lines
Objective. Use gradients to identify parallel and perpendicular lines.
The key idea
Parallel lines share a gradient. Perpendicular gradients multiply to −1, so the perpendicular gradient is the negative reciprocal.
Worked example
Find a line perpendicular to y = 2x + 1 through (0, 4).
- 1Gradient 2, so perpendicular gradient is −1/2.
- 2Intercept is 4.
- 3y = −x/2 + 4.
Answer: y = −0.5x + 4
Practice
Each question comes with a picture and a listen button. Try it first, then reveal the answer.
Common mistake. Using the reciprocal without changing the sign.
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