Math CentralQuandaries & Queries


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,

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