Perpendicular lines - Math Central

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Question from dinesh, a teacher:

we know that for two perpendicular lines
m1*m2 = -1
but this is not true for x-axis and y-axis?

Hi Dinesh,

You are correct. If line l₁ has slope m₁ and line l₂ has slope m₂ the l₁ and l₂ are perpendicular if and only if m₁ × m₂ = -1. The difficulty in applying this to vertical lines like the y-axis is that a vertical line don't have a slopes. A horizontal line like the x-axis has a slope of zero but a vertical line doesn't have a slope.

I hope this helps,
Harley

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